In my experience flange breaks have been due to poor quality spokes and/or improper building/truing. If I you start to see breaks around the threads expect more because fatigue is probably the culprit. Unless of course individual spokes have been tightened excessively to bring the wheel into true. I've seen some newbies try to true their wheels by tightening only. Too much tension in the wheel will cause spokes to break until the bulk of the stress is relived.

RunYun