whats the test kit you are using? API master test kit, bottlt #2 need to be shaken vigeriouisly to mix the chemicals.
and nitrAtes shouldn't of droped unless; inaccurate reeding of test, A water change has been done, You have vert heavy planting which is using the nitrate or lots of algae (which does the same as the plants).
the best e=bet is an Inaccurate reeding.
I have had this happen to me with an overly dirty filter that was allowed to run for longer than it should without a clean. I did this intentionaly though, as I was changing oer filters, and wanted a new set of media in the new filter. This isn't something that you realy want to happen though, as you need a very low flow through the filter for it to occur. Under normal conditions, with enough surface movement on the tank, there should not be anairobic conditions in the filter. Anairobc posckets elswhere in the tank probibly won't have a significant impact on the nitrate reading, unless you have a deap sand bed in a sump.