Mine was doing the very same thing for about a year now. From what I can gather the internals of the muffler break (rust) down in a way that causes this. Of course mine was after warranty.
No muffler will last forever but I was hoping for a little more than 4 years. I went back to my Cherry Bomb and some mods that smoothed things out. $50 on muffler (already had one) and parts looked like a better option than a new factory pipe.
Falling down is your bodies way of saying you just screwed up.