could be a bugged recap, you took the matriarch's ground spray attack, a ranged attack, and a dog bit in the same second (were about to take a heavy attack from one of the npcs too. This looks like normal, you may have pushed her too fast as well and they skipped the whole ground stomp sequence (which is an insta kill). I remember people always saying that the only dps race is the last island, never the first two. I can imagine that pushing too fast with that instakill mech can possibly cause this, even on normal.

This encounter is incredibly buggy, to give some perspective- I've killed the matriarch and all the adds one time and it didn't complete for me, the troll just kept infinitely spawning (the troll kept literally trolling me).