I am not very experienced at this so please take what I say with a grain of salt. If you were doing a dry hatch your air cells should have been plenty big for the hatch. If the eggs have evaporated enough during the first 18 days, increasing humidity to even 80% is not going to cause them to drown if they pip into the air cell since the eggs will not absorb moisture back into the shell. Did you check carefully to see that they did not pip into the air cell? If they pipped below the air cell then it could be possible that they drowned but not likely if they got through the shell. I would maybe suspect your ventilation being a problem, but again not sure if you took the blanket off before they pipped, there should have been enough air in the bator. HMMMM this one is a bit of a stumper. Wish I could be more help.