My 4" bird has the same problem from landings. I've flown her six times, and two of the six cracked a fin root on the trailing edge. The first time not so bad... Landed under main chute. The last time I flew it, pretty bad... I had a piece of tape holding the Kevlar blanket burrito'd around the main chute... Forgot to remove the tape. But I was careful enough to not wrap it, and literally just put enough tape to let it rip off under descent (I made sure I didn't even push the tape into the blanket...). Well, guess what? Tape stayed put. Main popped, but no chute... Just a burrito coming down.
From what I can gather, the upper section "staked" into the ground vertically, then the motor section was still horizontal, came down right on top of the upper section -- The damage indicates that the upper section collided with the motor section pretty aggressively. I don't even think I'll bother fixing this one. If I do, I'll just cut off the body tube above the motor tube (assuming there's enough space) and splice a new body tube over it. We'll see.
I should take pics sometime.
-Mike