I had a similar situation happen, only where my washer ties into the
drain is open to atmosphere. the washer tie in is of course a p-trap,
so gas should not escape....
the symptom i had occur was water would backup out of this p-trap, an
onto the cellar floor. the only reason it did not flow into the
washer was because the washer drain hose is not connected to the drain
The problem: Roots had reduced the flow through my sewer main under
the front lawn. So, under heavy drain use with showers, dishwasher
etc. the sewer connector was not letting enough flow into the street
system, the first place the sewer water could get out was my open-air
SO.... maybe you've got a sewer line problem with roots or other
blockage. That would explain why this is not a constant problem, and
why its a fairly recent manifestation.