Re: Washing Machine Drain Issues (possibly)
I suspect the source of the water is from the overflow tube and not the drain. Perhaps the level of your floor allows this to spill to the direction of the DWV and it runs along the outside of the pipe to the ceiling below.
Do you have a floor drain in this 2nd floor laundry (then perhaps the floor needs to be re-sloped to it?) or is the washing machine set upon a pan which is likewise set to a drain (which then perhaps the failure lies there - leak in that drain connection to the pan or the DWV itself serving that pan).
Many larger bedding materials, especially "comforters" for Full, Queen, and King Size beds can cause overflows of water and/or suds even in extra large capacity residential washing machines. Some more responsible bedding mfgs will label these products on the laundering instructions to launder in "commercial washing machines". They're usually referring to those super large capacity front loaders which use copious amounts of water and minimal amounts of soap. Sometimes this is labeled because agitation in a top loading machine might damage or prematurely weaken the fibers of the bedding material, or even damage it, but sometimes it is because it will potentially cause overflow or damage to the machine itself or the homeowner's home (water damage).
Usually the overflow is on the front just inside the case near the floor leveling leg on a top loading machine - the new side loaders it varies, ped mounts, etc.
That's my guess. Other possiblity might be suds backups only showing up with washing this comforter and backing up to the vent- and perhaps that's where your breach is.
I'd take the comforter in question to a laundrymat with a commercial super sized machine and run it through a few cycles using NO detergent and instead using a cup of distilled white vinegar in each wash cycle and nothing in the rinse cycle. Watch through the door and see if you have a lot of suds. Shouldn't harm the comforter, just get the soap out and make it extra clean and fluffy when its dried.