Well, the one difference I've had than you is that I've pretty much always run smaller tires on mine, significantly smaller (standard traxxas t-maxx chevrons). This takes away a lot of excess stress those stock tires put on things when landing, cartwheeling or hitting things.
My guess is that you did something just right (or wrong, depending on how you view it) to bend/twist the arm enough to put a bind on it as it was compressing so the end couldn't pivot as the shock compressed and caused it to snap.
You'd be surprised how much your arms bend when you hit your tire on something. I have some video with my newest camera that does 30 frames per second. I was amazed at how much things flex just when doing "normal" bashing when watching vids in slow mo. It truly is amazing that these rigs hold up as well as they do.
I'd replace it, but before making the order, take shock off the other side, take the spring off it, and compress it by hand and make sure it's not bent. You will need new oil as well, so make sure to get some when you get the new shaft.