Broken rear axle. Why does it happen and is it worth trying to repair? So I'm taking the time during the holidays to overhaul my commuter bike. The whole drivetrain was full of old oil, road salt and grime so I decided to take the whole thing apart starting with the back wheel. Once I got it off, I ended up with half of the rear axle in my hand and a bunch of ball bearings on the floor (I found them all eventually). 

The bike has been in use for about 18 months, and there were no rust and plenty of lube inside the hub so I don't really think it's natural wear.

What could have caused the axle to break? My theory is that they pulled the nuts too tight when they built the bike.

More importantly, is this a "user serviceable part" or do I need to get it repaired/replace the wheel? Seems to me getting the ball bearings to stay in place while I insert the axle would be quite challenging.
Most of times the axle is broken when not accurately jumping with bike (the same is with obstacles on road like holes and speed bumps). Wheels with freewheel are more exposed to this because their bearings (i.e. the place where wheel touching the axle) far from the place where the bike touch the wheel, therefor it have more moment to brake the axle.
To the question about worth repair. The only available repair of this - is to put a new axle, that means it will be like a new wheel (if you do not have other problems in your wheel). If you going to replace the axle, examine the bearings cup if it have any deformation. If it have, in most cases the wheel must be replaces. Examine also the cones.