Just thought I would share the information below, which was kindly provided to me by Dixondale Farms. Apparently there are many factors that can lead to onions bolting.
Onions and other commercially cultivated alliums are biennial plants, which means that they have two lives. An onion's first life begins at our farm, when we take it from a seed to a plant. When transplanted, the onion begins its second life. At a point when the plant has at least six leaves and experiences an extended period of cooling temperatures, it can go dormant a second time. As the temperature rises, the onion tries to start growing again, marking the beginning of its third life. The plant believes that it's going to die, so it tries to reproduce and grows a flower. This flower formation is called bolting or vernalization, and it's how we produce seed. Occasionally other factors, such as damage by cultivation or excessive stress, may cause bolting. That's why only a few plants may bolt in an entire plot or field. Should this occur, the onion will still be perfectly edible; however, as the seed-stem gets bigger, the ring associated with it will become piffy and inedible. If left to maturity, this ring will rot quickly and cause the entire onion to rot as well. It's best to eat the onion as soon as you see the seed-stem. Don't bend or break the top; the leaf is hollow, and breaking it will allow water to go right into the center of the onion and cause it to rot. While it is impossible to control the weather, planting at the correct time for the variety in question is the most important factor to limit premature bolting. Over-fertilizing can also contribute to bolting - if onions are too vigorous, too early in their development, bolting can result. Onions bolt as a reaction to cold weather stress. Temperatures under 45F may cause the onion to bolt when the plant has five or more leaves. Some onions are more or less susceptible to bolting than others and the process is not completely understood. Unfortunately once the onion does bolt, the quality of the onion bulb deteriorates rapidly and it should be harvested and eaten as quickly as possible.