Thinking back on it (a decade or more, so please forgive any memory lapses), we often used to notice a "hot cabin air" smell when someone (including automatics) overdid the demand for cabin heat. We used to think it was "just a little oil" condensed in the ducting, and being evaporated off by high duct temp.
Top of descent will call for engines back to idle, but at the same time enough bleed pressure to pump the cabin down, and possibly high cabin heat demand too. On some engines a high/low bleed system will go into high, and hot relatively high pressure bleed air will be run through the ducting, evaporating any volatile condensates in the ducting, even before the aircon packs, and possibly downstream too.
If there are organo-phosphates in the oil these could well be released into the cabin in this way. I don't know enough about organic chemistry to have an idea of evaporation or condensation temps, but I would not be surprised to find both in the operating range of of aircraft press/aircon systems.