This is Splash Damage, son, and in Splash Damage games, grenades can be cooked. You don't need to be waiting - you can be moving at full speed, and suddenly someone tosses a perfectly timed grenade, to explode in your face.
You really don't want to do this if I'm standing at your toes, do you?
But yeah, I'm aware of the possibility of cooking grenades. However, I'm also aware of the lack of ability took cook grenades properly by most people. Hence why the CoD-crowd prefers Semtex so much over Frag.