The M5 did cut off the yellow M3 (intentionally or not, it's hard to tell). However, the M3 was driving way too fast and passing on the right. That aside, when cut off by the M5, the yellow M3 should have braked and allowed the M5 to pass. Instead, the M3 sped up and turned left, cutting off 4-5 lanes of traffic and smashing into a few cars. Complete douche maneuver.
They're both morons, but the yellow M3 is real culprit here. He got mad because the M5 cut him off and he proceeded to act like a complete idiot.