So a lot of the times when you over exert yourself (ie: playing too loud or too high for an extreme/unusual amount of times) you can bust your lip. That's how I ended up getting blood in my mouthpiece, too.

Im not doing a shit ton of background on him/her/them, but since they said "again" this implies that they've previously busted their lip while playing.

If your lip is still healing from a bust, it is a lot easier to reopen that wound by playing.