Washing Machine Drain Overflows When Someone Is In The Shower," I Think"

[driz]

I have wondered why my washing machine sometimes overflows back out of the drain rarely but enough to be irksome. I never could figure out why as it seems to drain nicely whenever anyone stands around by it but nonetheless it sometimes dumps a couple gallons on the cement floor.
The machine is located in the basement of ranch style house and located right next to the stand pipe with both shower drains located right above within 15' on the ground floor above. No special hookups here just regular elcheapo Whirlpool and everyday water saver showers upstairs. The washers drain hose has a holder and is fully seated in the pipe and normally drains quite well.
I wondered long and hard what caused this just sometimes but today happened to be there when my wife was in the shower and it started to drain. The water came pulsing out like it was under some fairly stiff pressure back out of the top of the washers drain line. Of course now that I know what "seems" to be causing it things bear a bit more watching but is for now can someone explain what is causing this?

[HayZee518]

could be a partial blockage in the main drain waste line. the effluent is all gravity feed from the stack to the street. have you snaked the waste line? does your washer drain have a trap in it?

[driz]

Yea I will try to snake out the drain, all 5 or 6 feet of it. The drain should be ok though it has only the tiniest degree of slope due to a ledge being there. It has worked fine for 15 years though so I doubt that is is. What is a trap in the washing machine drain, I never heard of it? Mine just seems to have a straight out corrugated hose................

[HayZee518]

the pipe the washer drain line goes into and connects to the main waste line usually has a trap to protect against sewer gases entering your house.

[earthworm]

All of these traps work as dirt and crap traps as well..
They must be cleaned every so often, a filthy task if ever there was one !
And the traps should have a clean out plug.

[Redwood]

If your washing machine is on a separate line leading to the main drain than your shower then the clog would be in the main.