I am not sure there is one definitive answer about the flowering. Onions typically flower the second year if you leave it in the ground, but that isn't your issue :D.

Since you grew from seed and not sets, it seems the most likely issue would be the proportions of fertilizer you used (?). If you added fertilizer for tomatoes for example, your plants would want to put out flowers. If you did add fertilizer then check the numbers. A high second number would logically lead to more blooms (hence flowering)

If any of this relates to your garden, you could add extra potassium booster additives or buy fertilizer with a larger last number for next year (8-8-14 for an example *that is not a recommendation just a view of how numbers might look on the package*). You could also test your soil to see if this was the issue.

Other things that CAN cause flowering to be more likely would be using large onion sets. If possible ones smaller in circumference than a dime should be used for keeper onions.

When big onions are flowering, they will not keep over the winter. So eat them up :D. If they begin to flower after they are fairly large, pushing over the stalk and pressing it to the ground (and even walking on it) will sometimes get that energy back into the bulb where you want it.

Hope that helps a little. In gardening there is often no ONE answer to the problem.

No offense to Sami k but I think your fertilizer is the least likely culprit to your flowering onions.
For 15 years i've been growing onions for sale and i've found they flower their first year mostly for one reason:

Stress. This can be poor soil, bad drainage, not enough water, too much water, weed pressure, hot/cold extremes....you get the picture.
Having a few in your patch go to seed is one thing, but all them going means something went wrong.
If everything went perfectly and they still bolted you can curse me and maybe look into other varieties that are better for your area (though candy is a good onion, I had a lot of them bolt too. We had them planted in a poorly drained area and it rained non-stop this spring).
good luck

The surest way to make onions bolt is to expose them to near freezing temperatures once they've grown larger than the thickness of a pencil or so. That triggers them to think they've been through a winter and are now 2 years old, which makes them flower. You may have transplanted them too early (exposed to cold) or started them too early (grew too big too soon).

Large sets may already think they are two, or grow fast enough that they get sufficient cold exposure in early spring.

Here we sow onions in the fall and they grow slowly all winter until spring, at which point they grow fast and finish by mid-summer. The timing of when you plant them is critical, too early and they will grow too large before spring ends and bolt, too late and they will be too small when bulbing starts. The 'magic moment' is around mid-October, but this varies with climate and onion variety.