Hard to say from the information given - there's lots of reasons that could happen.
What's the 24 hour traffic report like in local news? How much of that can definitely be attributed to your own pilots?
Does your faction control all the dockable stations in the system? If not, how conveniently positioned are the ones you don't control?
Are you in a Powerplay sphere? If so, is your government type favourable, neutral, or unfavourable?
Is the % drop going mostly to one other faction, or is it spread across all of them?
Are the economy and security sliders also moving unexpectedly (for you or for other factions in the system) or are they stable?