many things can cause high nitrates. poor biological filtration... ineffective protein skimmer, high bio-load, over feeding, poor circulation in the tank, crushed coral substrate.. Im currently finding out even bioballs can cause it.
forgot to mention a lack of water changes. eventuily even with low bio-load and good biological filtration your nitrates will creep up from lack of water changes. Theres nowhere for the nitrate to go so it builds up...