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This has not at all to do with bash, but it depends on the completions programmed in the package bash-completion.

From some comments in the file /etc/bash_completion.d/mount:

# mount(8) completion. This will pull a list of possible mounts out of
# /etc/{,v}fstab, unless the word being completed contains a ':', which
# would indicate the specification of an NFS server. In that case, we
# query the server for a list of all available exports and complete on
# that instead.
#

# umount(8) completion. This relies on the mount point being the third
# space-delimited field in the output of mount(8)
#

Also, you find in the main file /etc/bash_completion the following comment, that explicitly talk about mount and umount commands:

# A meta-command completion function for commands like sudo(8), which need to
# first complete on a command, then complete according to that command's own
# completion definition - currently not quite foolproof (e.g. mount and umount
# don't work properly), but still quite useful.
#

Update:
The comment about mount and umount commands was removed from bash_completion in the commit:

_command_offset: Restore compopts used by called command.

This fixes completions that rely on their compopts, most notably
mount(8).
Fixes bash-completion bug #313183.

Released in bash-completion 1.99

This has not at all to do with bash, but it depends on the completions programmed in the package bash-completion.

From some comments in the file /etc/bash_completion.d/mount:

# mount(8) completion. This will pull a list of possible mounts out of
# /etc/{,v}fstab, unless the word being completed contains a ':', which
# would indicate the specification of an NFS server. In that case, we
# query the server for a list of all available exports and complete on
# that instead.
#

# umount(8) completion. This relies on the mount point being the third
# space-delimited field in the output of mount(8)
#

Also, you find in the main file /etc/bash_completion the following comment, that explicitly talk about mount and umount commands:

# A meta-command completion function for commands like sudo(8), which need to
# first complete on a command, then complete according to that command's own
# completion definition - currently not quite foolproof (e.g. mount and umount
# don't work properly), but still quite useful.
#

Update:
The comment about mount and umount commands was removed from bash_completion in the commit:

_command_offset: Restore compopts used by called command.

This fixes completions that rely on their compopts, most notably
mount(8).
Fixes bash-completion bug #313183.

Released in bash-completion 1.90

This has not at all to do with bash, but it depends on the completions programmed in the package bash-completion.

From some comments in the file /etc/bash_completion.d/mount:

# mount(8) completion. This will pull a list of possible mounts out of
# /etc/{,v}fstab, unless the word being completed contains a ':', which
# would indicate the specification of an NFS server. In that case, we
# query the server for a list of all available exports and complete on
# that instead.
#

# umount(8) completion. This relies on the mount point being the third
# space-delimited field in the output of mount(8)
#

Also, you find in the main file /etc/bash_completion the following comment, that explicitly talk about mount and umount commands:

# A meta-command completion function for commands like sudo(8), which need to
# first complete on a command, then complete according to that command's own
# completion definition - currently not quite foolproof (e.g. mount and umount
# don't work properly), but still quite useful.
#

This has not at all to do with bash, but it depends on the completions programmed in the package bash-completion.

From some comments in the file /etc/bash_completion.d/mount:

# mount(8) completion. This will pull a list of possible mounts out of
# /etc/{,v}fstab, unless the word being completed contains a ':', which
# would indicate the specification of an NFS server. In that case, we
# query the server for a list of all available exports and complete on
# that instead.
#

# umount(8) completion. This relies on the mount point being the third
# space-delimited field in the output of mount(8)
#

Also, you find in the main file /etc/bash_completion the following comment, that explicitly talk about mount and umount commands:

# A meta-command completion function for commands like sudo(8), which need to
# first complete on a command, then complete according to that command's own
# completion definition - currently not quite foolproof (e.g. mount and umount
# don't work properly), but still quite useful.
#

Update:
The comment about mount and umount commands was removed from bash_completion in the commit:

_command_offset: Restore compopts used by called command.

This fixes completions that rely on their compopts, most notably
mount(8).
Fixes bash-completion bug #313183.

Released in bash-completion 1.99

This has not at all to do with bash, but it depends on the completions programmed in the package bash-completion.

From some comments in the files /etc/bash_completion.d/mount:

# mount(8) completion. This will pull a list of possible mounts out of
# /etc/{,v}fstab, unless the word being completed contains a ':', which
# would indicate the specification of an NFS server. In that case, we
# query the server for a list of all available exports and complete on
# that instead.
#

# umount(8) completion. This relies on the mount point being the third
# space-delimited field in the output of mount(8)
#

Also, you find in the main file /etc/bash_completion the following comment, that explicitly talk about mount and umount commands:

# A meta-command completion function for commands like sudo(8), which need to
# first complete on a command, then complete according to that command's own
# completion definition - currently not quite foolproof (e.g. mount and umount
# don't work properly), but still quite useful.
#

This has not at all to do with bash, but it depends on the completions programmed in the package bash-completion.

From some comments in the file /etc/bash_completion.d/mount:

# mount(8) completion. This will pull a list of possible mounts out of
# /etc/{,v}fstab, unless the word being completed contains a ':', which
# would indicate the specification of an NFS server. In that case, we
# query the server for a list of all available exports and complete on
# that instead.
#

# umount(8) completion. This relies on the mount point being the third
# space-delimited field in the output of mount(8)
#

Also, you find in the main file /etc/bash_completion the following comment, that explicitly talk about mount and umount commands:

# A meta-command completion function for commands like sudo(8), which need to
# first complete on a command, then complete according to that command's own
# completion definition - currently not quite foolproof (e.g. mount and umount
# don't work properly), but still quite useful.
#