I was interested too and went and watched the last 10 minutes of the match, and here were the red cards.
84' - With play stopped and a player down in the box, away from the ball, deRoux for Minnesota grabbed the ball and was walking back towards his own goal with it and Nunes didn't like that. Surprisingly similar to an incident in Portland earlier this year when a Minnesota player did the same thing and resulted in a scuffle (but no cards), but they got into a bit of a scuffle this time too and Nunes may have contacted deRoux in the face, it was really hard to see though. deRoux got a yellow, Nunes got a red.
87' - After what looked like a fair non call, Pulido said something to the ref and got a straight red.
89' - Ramirez made a hard tackle just outside the box (to the side), may have gotten the player with the trailing leg, was already carrying a yellow, so got a second yellow and the ejection.
Bruno Menezes for Miami may also get a suspension for confronting the referees after the match. It was not pretty, and will be interesting to see what the result of all this is.