Your broccoli is bolting - this is a gardening term used to describe cabbage, cauliflower, lettuce and broccoli that goes to flower instead of making heads. The cause is warm weather and the cure is an organic mulch like compost on top of the soil to keep it cool or even a shade covering to keep the hot sun off the plants. Next year, plant the crop sooner so it has cool weather in late spring to set heads.