Drain out from washing machine is overflowing

Liquid plumber or just use a snake to clean it out yourself.

Glad you got it fixed quickly.

Washing machines pump out alot of water in the drain mode. I usually just call a plumber when that happens.

Curious how much did he charge?

If you use Drano and a short snake and it recurs in 5-6 months, there is a blockage farther down the line.
Found that out and after having a plumber run a snake about 80 ft down the line 2 years ago.
Have not had a problem since. He charged $125 at that time.

not a plumber so always wondered why washing machine drains were plumbed like that in the first place: why isn't there a "hard" connection that can be undone for clean out but protects against overflow?

My Speed Queen is doing it-soapy water during rinse cycle. Not a problem in 2 other houses so I assume a pipe issue. Not a problem if we do a second rinse. Any idea why?

I bought a pressure washer pipe jetter on Amazon for about $50 a few years ago. Works pretty good for soap, grease, ect build up in the pipes

I thought my washing machine had sprung a leak by finding water on the floor periodically until one day I was sitting at the kitchen table when the machine went to drain mode and I heard the water back up the pipe and come out the wall. So I went and bought some of that nuclear strength sulfuric acid drain cleaner, poured down there and waited 15 minutes and flushed water behind it and Wham-O its never been a problem since. If you use that stuff make sure you wear gloves/ face shield and use eye pro.