What makes marijuana buds grow?

For outdoor plants it is clear that it will take a natural route and grow according to the time of year. For indoor plants the grower has to make the plant think it is in a certain season. For example when the plant is in its germinating stage it must be under "Spring like conditions". ie. higher than average water, warm atmosphere, high humidity, what have you. But for the later stages of its growth an uninterrupted light schedule must be made.

But to more thoroughly answer your questions comes at the end of the process, the flowering stage. 12 hours of light, 12 hours of uninterrupted darkness, and "fall" like conditions are ideas to make the buds form.