What is the reason behind my newborn having bleeding in his brain?

Hard to say: A small percentage of newborns will have a tiny,usually asymptomatic bleed. More symptomatic bleeds would likely occur but vitamin K is routinely given to most within hours of birth. Premi's are more likely to bleed because of frail blood vessels. You did not provide specific information on your kids case, which might have allowed a more specific response.