Could be any number of reasons why Broccoli will "button" i.e. form tiny flower heads, Primary reason is usually the the plant was stunted somewhere alon it growth cycle, I have seen plants kept in theier trays too long and actually button before transplanting. Sometimes a cold snap will cause the same phenomena in the garden, Cabbage root maggots and root diseases will also cause this, If your plants are growing well, then harvest the button and hope for more normal side shoots. Once the flowers open up the plant will divert all its energy to producing seeds,