It's the way your current locale defines collations/sorting rules that's causing it, and how -kN uses field N to the end of the line when comparing lines, not just field N (And some locales will sort bc,pe before b,foo if they ignore the commas).
Use -k1,1 to only use that specific field, or specify the "C" locale and you should get the expected results:
$ LC_ALL=en_US.utf8 sort -t, -k1 test.txt
a,foo
ant,baz
bcd,ty
bc,pe
b,foo
c,bar
cn,cn
$ LC_ALL=en_US.utf8 sort -t, -k1,1 test.txt
a,foo
ant,baz
b,foo
bc,pe
bcd,ty
c,bar
cn,cn
$ LC_ALL=C sort -t, -k1 test.txt
a,foo
ant,baz
b,foo
bc,pe
bcd,ty
c,bar
cn,cn
