So when you pulled the corpses out of their pots, what did you have?
If the stem just ended, sep. from the roots, that would be dampened
off, and you killed with too much kindness - over watered. I have
some melons headed that way.

If on the other hand you have a root system attatched to the body,
then I'm stumped. Could be the soil has developed a microbe prob., or
the plants were not healthy to begin with, etc. Too many variables!
Your seedlings should just about dry right out between waterings. Not
desert dry, but dry, so as to not be waterlogged and develop healthy
roots.

But this is all just a wild guess - better luck next time!