I broke a spoke on the rear of my Raliegh hybrid at about 800 miles. I don't know what caused it. I didn't hit anything, or run in a pot hole, but I am a solid 300lbs. The LBS replaced the spoke, and did a really bad job of truing(way over 1/2" lateral) it wasn't much better than when the spoke was broken. I straightened the wheel by loosening the high spots 1/4 turn, and tightening the low spots, to get it passably straight without changing the overall tension much. My question is: the rear wheel seems to have considerably less spoke tension than the front wheel. Could this be why the spoke failed in the first place?
With geared bikes the rear driveside (right) spokes normally have the highest tension on the bike and the rear non-driveside spokes have the lowest tension. The front wheel spokes should be equal on both sides with tension a bit less that the driveside rear. The tension differential on the rear wheel is due to the asymetrical hub flanges to make room for the cassette or freewheel cogs. With conventional 8, 9, or 10-speed wheels the non-driveside spokes will have about 65% as much tension as the driveside spokes. This is necessary to center the rim.
Loose spokes break due to excessive flexure, usually in the "J" bend at the hub.