Occasionally, models for all mobs in the player's simulation distance will cease movement, not even wandering aimlessly like they normally would. These mobs will be in this state for an indeterminate amount of time, or after you unload and reload the chunks they're in.

While the game is like this, mobs will:

• still be interacted with by the player as if all was normal (e.g., attacked, ridden, fed).
• still make their standard noises (e.g. hurt noises, random noises, and when attempting and direct a mob, walking/clopping).
• still attack, melee or otherwise.
• If the gamerule mobGriefing is on, still interract with the blocks around them accordingly (sheep still eat grass even if their heads don't move, creepers still explode when near players for their respected time).

Some other observations:

• I have not seen this happen with players, only mobs.
• This affects singleplayer only as far as I'm aware.