There are several environmental factors that promote renewed growth of ripening buds. The most important is temperature. Resprouting often occurs in buds exposed to warm conditions, such as the heat directly under an unprotected HID lamp.
To prevent this from happening, keep a greater distance between the lights and the plants. Another option is to use light movers so the plants aren't directly under the light all the time and heat becomes more evenly distributed. Air or water-cooled lamp reflectors could also be used; they remove the heat generated by the light.
Another reason why your plants reacted this way might be that there is an abundance of nitrogen (N) in the medium that the roots are absorbing. Nitrogen promotes vegetative growth as well as new flowers.
Light pollution could also be a cause of new growth. Interrupting the dark period for even a few moments can reset the plant's hormonal clock. This usually is more crucial in early stages of flowering than near the end. However some varieties are more sensitive to light than others.
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