I think the remark that set him off was “stupid old douchebag”, and I think it was the “stupid” part that did it. Quasi is losing control of his mind, which understandably frustrates him. Makes him feel “stupid” because he can’t remember things the way he always has. Being called “stupid” probably felt like having his nose rubbed in it. I also think that word choice was intentional, needlessly cruel, and says much about the character of the asshat that used it.