i agree that axle is something that you (myself included) would take for granted wouldn't fail at your power/weight level but i guess it's just a reminder that metal can and does break. i don't think it necessarily has anything to do with strange they make a quality product. can't help but wonder if it would have happened to a mark williams though.
if you want to get an idea though of what that axle handles every time you leave the line start doing some math
engine torque x 2 (converter multiplication) x first gear ratio x rear ratio / 2
an example
engine 550 ft lb x converter ratio (2) = 1100 ft lb
1100 x 2.45 (727 first gear ratio) = 2695 ft lb
2695 x 4.56 (rear axle ratio) = 12,289 ft lb
12,289 ft lb / 2 axles = 6147 ft lb
so in this example every time you "hit" that axle it's handling over 6000 foot pounds (the actual "correct" term is pound feet) of torque
they do take some abuse
the collateral damage sure is a bummer though, like you said at least it didn't happen at speed