REASONS WHY explination:
The reason why is beacuse anime boys are perverts.
even when they son't want to be one they are.So anyway they get noebleeds for two main reasons.
Reason 1:the first reason is if they see a girl naked.
Reason 2:They see a realy pretty girl and think perverted thoughts.
Those are the two main reasons but theres many más such as finding a girl in ur bed.Or even seeing a pretty girl.Also it could be like in naruto when they read perverted books.perverts could be just if they see a big chested girl.And it And tu know they always get beatup after being perverts (which as a girl i would beat them up too!)Well i hope this answerd that pregunta which i know más than half of tu already know.
THANX!!