Answer: It sounds like your broccoli may have been planted a bit late and began to "stretch" before its time. It is not unusual for broccoli heads to stretch and bloom if left uncut as the days get longer and warmer. You might try replanting for a fall crop. Broccoli seems to really like ripening in the cooler days of fall.
Another possibility is that you planted a "cousin" of broccoli such as broccoli raab. Some of these tend to shoot up and bloom like you described.
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