When you get an erection
that forces too much blood to your dick. It then explodes from the pressure. This generally happens when you get turned on beyond normality.
As two kids, Derek and Julian, sat on the sidewalk, bored, they saw a curvy mid-twenties blond female walk down the street. "Dude, look at HER! She's so fine," Derek said. "I'm getting a nosebleed," Julian responded. Suddenly, a loud boom echoed throughout the cul-de-sac and blood splattered all over the house across the street. "..dude, what was THAT?" Julian asked. "Oh, I just had a dicksplosion," responded Derek, all smiles.
When someone is really emotional and cries during every show/movie/commercial/etc they watch
Man I cried more than 5 times watching the series finale of The Office just now and I can't stop. I'm definitely a serial crier.