Looking at your picture it appears that the bolt that holds the comp on is bent. The top of the comp looks like it is a bit away from the frame. That is consistent with an impact.

Odds are the projectile did not break apart.

Have you ever had a flyer that you were sure was not your fault?

You say that this happened with lead projectiles. I bet they were cast and not swagged.

All it takes is a cast projectile that did not fill out the mold or had an air bubble in it. Obviously the weight would not be distributed evenly. So it will begin to wobble as soon at it leaves the rifling.

Similar things happen with swagged and jacketed projectiles as well but cast are the most common by a large number.