It is a boner or erection that occurs questionably, however unlike inappropriate boners in public "why boners" usually occurs when someone sees something that gives them an erection constantly, it can lead to a new fetish or a new gender depending on what they have seen.
That moment when you know you should have an erect penis but don't for one reason or another be it known or unknown and it makes you angry.
I can't get hard for this girl because she has a boyfriend. FUCK! Let me go make some shit up for the urban dictionary. "Why boner" is when you're confused and horny. I'll call this the "why not boner".
It is said of the situation where a person has the bad luck to make contact with his testicles against an undefined surface or object, intentioned or not.
Given the nature of the word, it is more appropriate to design cases where the interaction is made with a moving object, for example, a ball.
Although it is extremely painful for the victim, it tends to be considerably funny to people who witness it.
Today in the baseball game the pitcher took a nutshot; the baseball hit him in the nuts.
Man, I just watched the funniest nutshot video ever.