The most common cause of broken spokes is low spoke tension on the non-driveside rear wheel. The hub flanges on the rear wheel are asymmetrical to make room for the rear sprockets (s) and chain (but this should be less of a problem on a single speed). As others have said, most likely the wheel was not properly tensioned and trued. The rim did not cause the spokes to break, and neither did the heat. It's also possible that the spokes used on your wheel were not top quality. The rear wheel carries a much greater share of the weight, but your wheel should be able to carry your weight easily.