So here is my observation.....i had several plants growing in early preflower. Some were under the UV lights and some were under the regular LED lights. The UV plants certainly had early crystal production. I noticed that all of the plants under the UV had earlier crystal production. I proceeded to move the plants and with 2 days the plants that were under the non UV lights had a noticable amount of crystal forming.
The theory - UV is harmful to the plants(and us as well!!) The plants response to harmful light is to add more light refraction which comes in the form of mushroom shaped trichomes to break up the harmful light. To me it makes total sense and even without scientific proof I have already seen the effect first hand.