We now know what triggered the confrontation between Diddy and the UCLA coach -- eyewitnesses tell TMZ Sports that Sal Alosi embarrassed the rap mogul in front of the whole team.
Our sources tell us ... strength and conditioning coach Sal Alosi was riding Justin Combs hard -- and screaming out loud, "I don't care if your dad's here. This is UCLA. I'm going to treat you just like I treat everyone else."
We're told Diddy got super heated -- feeling as if the coach was trying to humiliate and embarrass both him and his son in front of the rest of the team.
Of course, Diddy and Justin went to meet with Alosi in his office following the training session ... and the rest is history.