I have a plant in flower now that is fox tailing... the comment below makes sense because I recently moved it from a 400 HPS to a 600 MH and the foxtailing began shortly after. 

Fox tailing is generally caused by heat stress near the buds... Hence why the top buds nearest the reflectors generally get this problem the worst... Foxtailing isnt a bad problem, but it usually is a sign of other factors that could be better controlled in the growing environment... For instance, heat near buds... That being said, certain strains have calyx growth that just leads to fox tailing more than other strains 

Source: Trikhoma C