Been lurking a long time and finally had something to add which fits with speculation by
L-38 above. On another board an AA AMT posted:
“Apparently one of the engines …starting leaking oil internally into it's compressor section and filled the cabin with a nasty OIL MIST. They are still trying to get all the oil out of the A/C packs and ducting, so the aircraft can be returned to service.
“Turbine engine oil is toxic. So making the passengers sit there and breathe it to save a few bucks on some emergency slides, makes it sound like they had a "bonehead" for a captain. It doesn't matter whether if it's a fire or oil mist, get the people out of there!”