Entities riding other entities teleport below bedrock when motion is applied

1. Summon a chicken riding a pig:
/summon Chicken ~ ~4 ~ {Riding:{id:Pig}}
2. Use the entitydata command to apply motion to the pig:
/entitydata @e[type=Pig] {Motion:[0.0,1.0,0.0]}

To quickly see the effects, teleport to the pig:
/tp @p @e[type=Pig]

Note: This does not occur if the pig is NOT being ridden.