It is my understanding that the hormone auxin is responsible for apical behavior-- or, in other words, the molecular tool for such behavior/ structure.
The accumulation of auxin in a branch , such as in the main branch in an apical plant, will cause that branch to reach for the sky while having all other branches support it in it's venture.
When topping your plants (removing the top node), applying the F.I.M. technique (Fuck I Missed = removing 80% of the top node), or applying LST (low stress training), the apical plant (such as cannabis) will reallocate the phytohormone (plant hormone) auxin to side branches,instead of to the main alpha branch, trying to maintain its apical structure. In other words, the plant is thinking its dominant branch is no longer able to be dominant and invests it's auxin in other contending (for dominance, or closest to the light) branches.
Read up on it, I think it will explain a lot:
http://www.plant-hormones.info/auxins.htm
http://www.thctalk.com/cannabis-forum/archive/index.php/t-17720.html
To answer your question, Patlpp, the popcorn bud is the result of a lack of auxin-- which typically means the lack of resources such as PAR (photosynthetically active radiation units).
atypical apical....
