Had it happen to me on dirt with a revenant, was clearing them out from a tribe in 941, when one of them seen me, ran at me like it had a rocket up it's butt, and hit me several times before I could react, I beat a hasty retreat to heal and when I came back the rev was back to normal.
Been chased down and killed a few times by bears, no idea what causes it but they sure can move way faster than I can when it happens.
Also recently had a couple of bears give me a pounding while trying to lure them back to my lab...er tribe :P, they lost interest in following me for some reason, agro'd and were on me before I could react.