It happened several times now, and it confuses me every time anew but also made me chuckle

Doing AK, standing in front of Demon Wall, hacking junks out of it with me axe it readies Repel, it uses Repel, I don't move an ilm ... suddenly the screen turns black, loading ball in the bottom right for a while and I reappear at the door of the fight.

What happened?