It's funny becouse I thought like "Could this be the enrage or something?" But it always happend when he was at 30%. Guess I should take different DPS with me then. Becouse, as healer there isn't much I can do
It can be (at times) difficult to problem solve with the same DPS. Sometimes you think a mechanic happens at X% but it could simply be that DPS does not change, giving you the erroneous idea that it's either i) bugged or ii) that it's a mechanic that always occur at X health levels.
From reading on the forums, some people report this "mechanic" occurring anywhere from 20 - 25%. I saw it in one run at about 18-19%. These varied numbers lead me to believe that it's an enrage.
IFF the reports all agreed on X% when it occurred, you can reason that it was a mechanic that always occurs. But because this is not the case, I'd lodge my bets on it being a soft enrage.