Are these onions from seed or from sets? Usually this is caused by poor growing conditions, too dry, then too wet. If onions get a setback, they think to themselves, "Quick, we'd better flower and set seed before we get dried out again." (so to speak!) Either that, or alternating warm and cold weather.
I always find a proportion of sets go up to seed prematurely, usually not many though. Onions from seed though a lot more trouble are far more reliable.