Here are the only ways in the code that obedience can be lowered without using items on the girl.
Girls addicted to drugs loose 30 obedience each week.
These traits, when lost, can lower obedience: Meek-20, Mind Fucked-100, Dependant-50, Masochist-30, Lolita-5, Nervous-10
These traits, when gained, can lower obedience: Iron Will-90, Manly-10, Tsundere-10, Queen-15, Princess-5
If a girl is raped by a customer -10
If a girl on security duty gets defeated -10
I can find no other reason your girls could be loosing obedience.
Nope, no changes in traits, no items taken away or added, no addictions
just sometimes when the obedience modifier reaches 100 it decides to become -9x in the next turn
(-100 + the change from the current turn)
when it would go above 100 if that were possible
lets say i take a girl who really hates me out of the dungeon (after 30 weeks there, they do hate you)
but who has obedience modifier close to 100 because of torture
then hate decreases and love increases over the next 50-60 turns in the player house
there is a small chance, that this bug happens (when love gets close to true love state or reaches it)
well this bug is quite rare and easy to fix, i just edit savegame and delete the -
so it's not really important
searching for the cause of it will take much more time than the 30 sek savegame editing every few days with the workaround