I've seen this too with plastic tipped bullets. I've always thought that over time most plastics will become brittle unless there is a reinforcing element introduced durring manufacturing. I've also thought that the harder the platic, the more brittle it can become. Maybe it's due to varying temperatures. Or it could be due to the breaking down of the petrolium products used to make plastics. I do know that UV light will break down plastic in a hurry as well. But this does seem like a problem that Bullet manufacturers should and could address.
On another note, My STW has a Vias Brake. It came with it, However the rifle is heavy enough to shoot comfortably with out it. The cool thing is the extremely low recoil allows me to see the bullet hit the target as there is little or no muzzle rise.
Dan