I may have caused that--I never know what to do to take care of puparia. It seemed to be accumulating condensation around it, so I did not want it to drown. I used a fine paint brush to brush away the water. Perhaps I should have left it alone...

Sometimes that just happens for no apparent reason, and it's definitely a good idea to keep puparia from becoming submerged. If there was just the one puparium (I didn't see any others), it would have been better to remove the plant material from the vial to prevent mold etc.--the leaves weren't moldy but they were smelly, and the gases produced may have been part of the problem.