Its the body activating its "fight or flight" response, basically all the blood going to the most "needed" places and unfortunately our cock isn't one.
Totally get you though, playing basketball with a guy and watching it go from semi hang to pointy nub. The flip is extreme guarding and instant hard ons all around. God I love sports
It means your body's saying, "Sorry, Mr. Penis, I need the blood elsewhere right now so I'm leaving you totally empty. You can have it back later, as much as you want!"