I have observed this happening in 1.4.5 as well, though not nearly to this extent and usually AFTER going down a block, not before. My theory has been that their pathfinding is trying to take them along the small gap they 'skipped' from the forward momentum affecting their fall, thus doubling back, but have not yet tested this in depth. Looks like pathfinding was tightened up for some reason.
As already noted, spiders are largely unaffected. Endermen also appear unaffected, as they do not jump, but also appear to always have their mouths open.
I have observed this happening in 1.4.5 as well, though not nearly to this extent and usually AFTER going down a block, not before. My theory has been that their pathfinding is trying to take them along the small gap they 'skipped' from the forward momentum affecting their fall, thus doubling back, but have not yet tested this in depth. Looks like pathfinding was tightened up for some reason.