how does matchmaking work (firefight)

I’ve had this a few times now where i’ll get into a fresh match and play through the full game with the people in it and we’ll carry on for a few games back to back and it all goes well untill suddenly im dumped into a game in progress and the people i was playing with… just gone

I play on the expanded search mode but why does it randomly remove me from the existing bunch of players and stick me into games in progress? why not just keep the group rolling?

Because you were the “lucky” one. There was a spot available in another game due to someone quitting, and you were chosen to fill it. That’s all there is to it. No need to think any harder than that.

Easy fix is to party up with the guys you had been rolling with. The game will never split you up in that case.