When having sex for the first time in a while (usually because you are a loser) and the debris in your pubes "crumble" and fall off/out of your pubes. Debris such as dandruff, dust, mold, dirst, and in some cases dried cum from uncontrolled masturbation events.
Girl 1: So i heard you had sex last night
Girl 2: Yeah but it was obviously the guys first time in a while
Girl1: Why, did he crumble?
Girl 2: He crumbled the whole time!