I really can't tell for sure, but the "usual suspects"
[Side note: anyone remember the statement "Round up all the usual
suspects" from the movie, "Casablanca"?]
for sprouting mold are:

too wet, too much light early on in sprouting, or the seeds have the
spoilage bacteria on them.

You can experiment with the first two above. If that does not work, you
can try disinfecting the seed with a short soak in a weak bleach + water
solution. (Even if you won't eat such seeds, it may help eliminate a
possible cause.)

One other possibility: germination rate too low - too many seeds are
unsprouted and rot. This can be a major problem with alfalfa.

Tom Billings