Warning: this is entirely my own thought, I have no evidence whatsoever to back it up.
In evolutionary history, the one time our ancestors would have had to concentrate the most is in hunting. While waiting for the prey to get close enough to kill it the hunter would usually be motionless and fixated on the prey (watch a cat hunting). I find it reasonable that, just before the actual strike, we would hold our breath. It would remove the last possible clue to the prey and might allow a more accurate throw. So, possibly this pattern of behavior has remained: any intense concentration might trigger the 'stop breathing' response as well.
