i started a world in 15w47c (latest at the time) and had this problem with chickens when i was trying to get them to my chicken farm using seeds to get them to follow me. when there was a one-block high tall grass next to a block the chickens would jump and spin until i pushed them out of the grass. not sure if it is the same with the 2 high tall grass or anything else or any other mobs.

created world and only played in 15w47c