A lot of crops, including beets, spinach, cabbage, and lettuce, sometimes form seedstalks too early. The quality of the edible portion of the plant goes to pot and there's nothing to do but plow it under. Fortunately, we've studied bolting in lettuce quite well. We used to say it was caused by high temperatures. But if you've gardened for a while, you'll know that is too simple of an answer.
Nothing is that simple.
Yes, high temperatures are a contributing factor, but the length of the day and the cumulative number of hours of light are also important players. And the more long days you have, the faster the process moves. Now, in the gardens of our region, long days and high temperatures go hand in hand, so there you have it. To further complicate things, the high temperatures can bring on moisture stress, which itself may speed bolting. And the amounts of bitter compounds, the sesquiterpene lactones, increase as the plants approach flowering.