Reason? You gotta wait after you hear the 1st 'explosion' coz he will follow up with most probably another 2 rounds and if you're in the midst of changing his diaper/nappy, good luck being in the 'firing zone'!!!
Needles to say, Grandma, confinement lady and yours truly has had run in's with him... :)
Either that, or he just wanna have some fun.
So the advice? Wait for 10mins after the 1st explosion.