Pantsing Scene and Drew's Tighty Whities

When Drew is pantsed at the beginning of the movie, how exactly is Trevor able to pants him. It looks like Trevor just rips off Drew's belt and Drew's pants instantaneously fall down. The problem with that is the fact that martial arts belts are not actually used to secure pants the way that a regular belt is. They are more like a symbol of how advanced you are in martial arts. Most people who I know wear them above the waist, so they could not possibly be used to secure pants. As a black belt, you would think that Drew would know this.

Were Drew's pants simply too big for him? Was he actually foolish enough to try to secure his pants with his martial arts belt, mistaking it for a regular belt, instead of looking for a pair of pants that actually fit him better. I've seen a lot of people say that, in real life, Trevor probably would've been disqualified for cheating, but all he did was remove Drew's belt, which, in martial arts, should not have caused his pants to fall. Or did Trevor do something else to make the pants fall down that I simply missed?

Also, why was Drew wearing tighty whities? In martial arts, it's typically required for guys to wear protection, which means he should've had on a jockstrap or a cup. Again, you would think that as a black bet, he would've been well aware of that rule. Why did he decide to just wear regular underwear? And, if he absolutely felt that he had to wear underwear instead of proper protection, why go with tighty whities? You would think he would've worn boxers or boxer briefs so that he would be less embarrassed in the event that his pants did fall down in front of the crowd just like what ended up happening.

It's easy to feel bad for Drew as he crawls out of the ring with the entire audience laughing at him, but it seems like he was, in many ways, practically asking to be humiliated in front of the crowd.