That failed in flexure -- in other words it had a side load, or perhaps it buckled. The side with the "curl" was in compression. The more flat side of the break was in tension. So, for some reason it got flexed into a curve and broke.
Ok so when you had the cylinder out do you see a rock or something down the hole where the cylinder comes out of? Did you check to see if the cylinder is binding at any point from full retraction to extension?
Now I'd have to see the whole thing to be certain but I'm not so sure I like the design of that cylinder enclosure. It looks like a good idea but where does the debris that falls into there go? Can it fall out again?