Stress cause hermies, that and bad genetics. You can stress you plant by messing with the light schedule or having a light leak in your room. You can stress them in other ways too but this is the most common issue. Trimming fan leaves is not likely to be a factor although you don't want to over do it. There is no reason to trim unless leaves are dying, or if they are blocking light from bud sites and even then there is debate on this issue.