There were multiple enemies. The titan you engaged was a sentinel, but you can clearly see a pulse grenade kills your teammate beforehand. That pulse grenade was thrown either by a stormcaller or striker. After you killed the sentinel with nova bomb, you got hit with an aftershock from fist of havoc. Chances are there's a good amount of lag involved. Also highly likely is that the striker titan was glitches through a wall or terrain so he wasn't visible