Basically, because this sort of thing would happen otherwise.
I've called Dinesh D'Souza the Scrappy Doo of apologists, and I stand by that while extending it to political debate as well. He's animated and excitable. He's cocky. He's on the small side, both in terms of notoriety and stage presence. He, frankly, seems like the sort of guy who isn't going to do all that well in a dustup.
Then remember that this is a guy who managed to hand not just Daniel Dennett, but Christopher Hitchens their heads in an onstage conflict.
Because Scrappy Doo there, for all his excitability and energy and seeming naivete, is crafty. And not just crafty: relentless. Other debaters will usually be content to leave their opponent with a question they can't answer, and hope that the lingering sense of puzzlement in the air will suffice to make their point. They are satisfied with drawing blood. D'Souza prefers to go in for a kill when the opportunity is there. And in this case, it was.
Now, it's easy to watch this and feel a bit bad for the student. Yes, he got rolled. He was left standing up there, dumbfounded, clearly looking for help and coaching that was never going to arrive. What could anyone say? That no, he deserved to keep his privilege?
When you realize that SJWs are typically made up of well-off white children, you can easily see one reason why a frantic avoidance of engagement at any cost is sanctified, blessed and encouraged.
No one wants to be this guy. And yet, barring a severely stacked deck, this - or some form of it - is easily what they can become if they argue with the wrong person openly.