When a mob is spawned in riding another mob and you change certain NBT of the mob on the bottom both mobs will freeze and then a few seconds later the one on the bottom will despawn or die.

The mob that was riding the other one will remain still, if the NBT changes while the 2 mobs are falling through the air they will float as if the NoAI tag has been used.

An example of this is summon a Spider Jockey into the game.

/summon Skeleton ~ ~10 ~ {Equipment:[{id:bow},{},{},{},{}],Riding:{id:"Spider"}}

and then run this command:

/entitydata @e[type=Spider] {Equipment:[{},{id:minecraft:bone,Count:1b}],DropChances:[0.0F,1.0F]}

If the bottom command is applied to a normal spider where nothing is riding it then everything is fine but you apply it on a spider (or any mob) with something riding it an the issue occurs.

Hope i've made the issue clear, wasn't sure how to word it.