Hi. I'm in a 2nd floor apartment, 2 floors total. The 1st floor unit underneath mine has an identical floor plan. About a week ago the drain was a bit slow, I put some drain cleaner in the tub drain according to the instructions. The drain worked fine after that. I showered every day since then, so I'm pretty sure all that ran through. This morning I woke and found this disaster:

Google drive folder with pictures: [drive.google.com]

Something caused sewage to explode upward from the drain, leaving silt spatter 5 feet up the side of the shower insert and shower curtain, and all over my bathroom floor and toilet where the curtain wasn't closed all the way.

The company's maintenance guy said it was probably "vapor lock" that caused some kind of backfill pressure. Does that add up? It sounds to me like they're trying to avoid looking into a more serious problem.

Could this have been caused by the neighbors downstairs putting chemicals down the drain?

And why am I responsible for cleaning this?

The maintenance guy is full of it. To me it looks like maybe a pressurized drain cleaner was used downstairs and the building also has a venting issue.

Did they do any drain cleaning work that day when your tub gets soiled?

Could be a jet fed from downstream with those high pressure nozzles, or can be fed from the roof down.