Chaotic Surge

Chaotic surge from act 1 boss in is really weird to me it makes you get oneshot after having like 10 of them and its nowhere explained how u even get them. Is it him hitting me or me hitting him with chaos or what. Nowhere i could find any explanation. And it seems from recent patch it became so deadly. It oneshot me on HC 2 times already so im not playing hc till i get some info.