This is just a bug. A common one too, in fact; its more of a glitch if you ask me. This happens if you collide with the ground in a certain way (the wrong way) or if you hit the ground on 4x physwarp. Also, with this amount of physwarp, it occurs with a kerbal on EVA on Jool too; so if you mush land on jool with a kerbal on 4x physwarp, just dont try to get him to stand up, or this glitch happens.