If people are dropping like flies, they are dying or falling down in large numbers.

The heat was overwhelming and people were dropping like flies. 酷暑难当，人们纷纷病倒。

to stop doing an activity in large numbers

There used to be over 20 of us in our aerobics class but they're dropping like flies. 过去我们的有氧健身班有20多个人，但是现在很多人都退出了。