I can smell sewer gas when washing machine empties into basement drain. I cannot smell sewer gas otherwise. What would cause this?

I can smell sewer gas when washing machine empties into basement drain. I cannot smell sewer gas otherwise. What would cause this?
What you're smelling is a bubble of sewer gas that's being forced back through the trap when the washer discharge hits a partial blockage and bounces back sending a bubble ahead of it.
The solution would be to snake the drain line. Good luck, Tom