Bound bruisers have very speedy legs

I was doing a defence encounter (hard difficulty) and one wave had 2 bruisers and the next had 4. For all 6 of them, after they drop the hammer their legs go into super speedy mode (walking animation looks sped up) and they're frighteningly fast. Tried to record it but they almost killed me in the process. Also, are they supposed to be able to regen their health in full?

I don't know if it occurs in other realms because I stopped playing for the day after the return to respite from map crash and I don't want to run into anymore bruisers today.