If the grid fins were stuck in an unhelpful position, then the engine would have to compensate much more significantly, reducing the trust available in the 'slowing down' vector.

It's also likely that 'hit hard' just means it crashed. Looking at the pictures of the ASDS, it's clear the rocket came down off-center. Perhaps it was at the appropriate velocity, but because it landed on top of a bunch of support equipment, it fell over and exploded.