Mobs die when there's more than 33~ in a 2x1 farm drop V1.11

Suffocation is a new game mechanic that was added in 1.11. From the Minecraft Wiki:

• maxEntityCramming
  • Defaults to 24.
  • If one entity tries to push more than specified number of entities, it starts to suffocate.
  • Gets damaged 3 hearts by tick; passes though armor.
  • The death message is <Player> squished too much.
  • Setting to 0 disables the rule.
  • Affects all entities.

Basically, when there are more than 24 entities touching each other, and they can't move, they take damage. This essentially nerfs a lot of farms where lots of mobs are in the same area.

If you want to turn suffocation off, use the command (you will need cheats)

/gamerule maxEntityCramming 0