Most bosses have an attack where they rush forward indicated by a larger arrow showing the direction they'll be moving. Unlike other boss attacks, the hitbox for this move seems to become active the instant the attack starts with a hurt area considerably larger then the boss itself.
If a player is in the hurt zone when the attack begins, it's possible to get hit before the indicating arrow even shows up (even in single player/hosting). If the attack hits like this, the boss can instantly act again and potential start a new dash, which will hit instantly once more if the player was unable to get far away. I've had a boss do a dash 3 times in a row in less than 2 seconds. Possibly would have done more if I hadn't escaped or had enough HP left to actually live a 3rd hit.
I'm not sure if this is intended behavior, as all other boss attacks are clearly scripted and give a reasonable reaction window.
This is easily recreated by being near the front of a boss when it chooses to dash attack.