I've had this problem twice now and I'm actually not too concerned as to fixing it as I have a new (much better) wheelset on it's way right now, but I'm curious as to what did this.
My bike (was) a stock Raleigh 2.0 2006 mountain bike with pretty cheap parts all around, I'm not complaining as I got it new for $10 as part of a Dominos promotion. I noticed a problem a few months ago of the back wheel being slightly loose (like the ball bearing raceway nuts weren't close enough together) and eventually it became enough of a burden on my riding that I decided to fix it. I found upon dissasembling it that the axle had actually broken, bending the skewer with it, grinding down a spacer, and pitting a raceway.
I headed over to my LBS and got a replacement axle and raceway and bought a new skewer and reassembled it. While it may be partly due to my inexperience in reassembly, I think I did a fairly good job and the wheel was spinning properly and I installed it back on the bike.
Fast forward to my ride home tonight, where I feel my rear wheel suddenly get wobbly, I got off and checked it a few times but ended up just riding home on it. When I dissasembled it at night, the new axle (less than a week old, maybe 20-30 miles on it) was bent in a slight bow shape and the new skewer was a little off as well. The wobbling had been because one raceway was loose and backed out, but I can't help but suspect it was because of the bent axle anyways.
As I mentioned, this doesn't bother me.... it's a learning experience and I decided to upgrade my wheels and drivetrain completely only a night ago, so much better quality replacement parts are already on their way, but I am curious as to why the axle bent so quickly.
I am borderline clyde (just a tad over 200 lbs) and carry groceries/gear on my back (maybe up to 30 lbs) pretty regularly, but this is a mountain bike and I've never had problems with the tires or rims. I suspect that there may be a ball bearing missing from each side of the rear hub, which could do it I suppose, but it did not give me noise/stability problems early in the installation. I also do mostly street/light trail riding so I doubt it's due to pressure from jumping (which i don't usually do) or otherwise.
So any guess of what killed my axles?