I lowered my pressures to obtain a little better traction as I had done in June. Apparently the lower pressure in that tire allowed a rock from the 'rock slide' hill to bust the bead. And as you said the air went out. I heard it happening, which sounded pretty weird, but didn't know what it was and I was at a somewhat critical part of the hill and didn't feel like I could stop. I made it to the top and the pictures depict pretty much what I saw. I'm not sure if there is damage beyond the radial split, that was all I noticed too, and I felt like that was a fatal injury for that tire.
While I was soaping up the beads on the rim tonight, I found at least 1/8" really caked mud across the whole bead and stretching nearly 180* around that half of the wheel. This stuff was essentially concrete requiring me to chip at it with a putty knife to remove it. I guess I picked it up in June at Haspin when I actually spent some time in the mud holes. That, I would assume, weakened how well my bead was previously set. The mud was definitely not there when I originally mounted them either.
I haven't ever heard of that happening. I guess it is another reason to stay out of the mud pits, even though they are soooo fun!