Why do the caps fragment and get stuck in the cylinder jamming the action?

They need to "splatter" so you are able to remove them from the nipple easily. So they are designed to be pretty brittle, but if they get stuck in the action, I'm guessing it could be the brand and substance of the cap; the metal used may be too brittle. If changing the brand doesn't help, I suggest taking a careful look over your pistol and seeing if anything is off, like something up with the action itself. Some cap and ball pistols have enough room for the cap to fall off between shots by having the weapon being tilted up before the next round, the used cap falling out before being caught in the action.