Probably a timing issue. Maybe there is the case when different biomes get different number of ticks for their objects (cause some biomes are being 'busy'?). Like some trigger that fires, when an object changes a biome and waits too long for some data. Maybe such triggers are universally used to apply effects (like stamina drain for a player or speed limit). When this happens to the player it just looks like a glitch and the cart just bumps into hist ass (unnoticeable).
Probably easier to reproduce on horse as it's just faster and lower amount of 'lost ticks' required to get far ahead.
Just wild guesses, probably bogus.