With GNU sed, you can match each contiguous string (i.e. without whitespace) terminated by : and then place a newline before all but the first one:
sed 's/[^[:space:]]\+:/\n&/g2'
If your version of sed does not support the gn extension, you can use a plain g modifier
sed 's/[^[:space:]]\+:/\n&/g'
which will work the same except for printing an additional newline before the first key. You could use perl -pe 's/\S+:/\n$&/g' with the same proviso (there may be a perl equivalent of the GNU sed g2 but I don't know it).