Since 1.8.2 pre4 hoppers grab items from above from minecarts, that are standing on the block next to the hopper. This doesn't affect all directions. When the minecart is facing towards negative z or negative x, items remain in the minecart. In all other direction the hopper grabs the items. This behaviour was different in older versions.

Players can deal with it. But at least it should be consistent in all directions. Orientation related behaviour is really annoying in Minecraft.

I also uploaded a video, to demonstrate this behaviour