Extended cold or heat can cause bok choy to bolt, or send up a seed stalk. The leaves develop a bitter flavor that renders them inedible after bolting. Although bok choy can withstand some light frost, an extended period of temperatures below 50 F can cause bolting, as can temperatures over 75 or 80 F. The seed stalk grows to about twice the height of the foliage and produces a small cluster of yellow flowers. After the flowers wilt, the seeds finish maturing in the remaining seed pods. Bok choy wilts and dies after setting seed.

http://homeguides.sfgate.com/growth-stages-bok-choy-plant-59557.html

Thanks, but that didn't answer the question I asked

Bok choy will always flower in summer when spring planted for reasons xiangirl mentioned. That's just what it does. As a food crop it does best, as do most all asian brassicas, when grown in the fall. There is nothing wrong with saving the seed from those that have bolted, assuming they were unstressed and grew well before bolting.

Rodney