gravity flat bar road bike (not the cheapest, not the most expensive)...
it has Alex rims, 32 hole spokes with 3x pattern.. (which I have other bikes that have 32 holes and have never broken a spoke on them)
I also had a Trek 7100 multitrack that had the same problem and basically, the shop told me that the style of bike was the problem (and of course I am a clydesdale).
supposedly, because of the flat bar, I put more stress on the back wheel.
However, I like the same position on all my bikes flat or drop bar (seat and handlebars need to be even)... so I guess I really didn't think that would be the problem.

oh and one other bit of information: it has disk brakes... don't know if that would make a difference...