Have you checked the rear hub carefully for signs of fatigue / failure / cracks?
I once had an MTB hub - Shimano Deore DX if memory serves - where I kept blowing through spokes in the rear wheel near the hub. I'm a lightweight, so it wasn't excess weight, hucking the bike, jumping off curbs or anything like that. I think it was on the third or fourth replacement that I decided to take a really close look at the hub.
Sure enough, there's a hairline crack right at the spoke hole that was stressing every spoke threaded through that hole, new or used, and eventually causing complete failure. I dismantled that wheel, threw away the hub (wasn't gonna help anyone now), and told myself to check all components very carefully when going through unusual wear patterns.