Standing on the edge of soul sand, grass paths, and other slightly lowered blocks causes the player to be forced off of said blocks if they are on the edge of a y-level. The player is forced to repeatedly jump forward to prevent being shoved off by the block. I do think this bug was already reported and resolved, but for the earlier pre-releases, and it's cropped up for me in the most recent one. 

Some observations: Mobs don't seem to be affected by the pushing

                                 If you are on a horse or donkey, the pushing doesn't happen

                                 Crouching does prevent you from falling off the blocks in question, but not from being pushed on them. 

How to recreate: Place any of the slightly lowered blocks on the edge of some terrain and try to ascend/descend them.