The answer to this may be down to light quality. Far-red light stimulates bulbing and if plants are densely packed and shading each other there will be a higher ratio of far-red light which would induce earlier bulbing.
Does anyone know if dirting around the onion can stimulate bulbing. In ginger this is done to stimulate the growth of the rhizome and to inhibit greening of the material. You onion people probably have some good insights. Paul Reed Hepperly
From previous studies, Changing the light quality and temperature without significantly modifying the photo- synthetically active radiation was found to affect bulbing. Rapid bulbing occurred under red (R)/far-red (FR) ratio ( from 0.67 to 1.37) . Also, the temperature at 21 0C was more promotive for bulbing than 26°C. No bulbing was induced at a high R/FR ratio (7.3) irrespective of temperature used. So, any factors affecting R/FR ratio will be affecting the bulbing. Also, at the molecular study, flowering and bulbing in onion are influenced by temperature and photoperiodic conditions which affect the expression of FT-like genes which could play roles in bulb formation
these articles help:
Journal of Horticultural Science. Volume 62, Issue 3, 1987