Why does this happen

Not enough info.

Size of stand pipe, height of stand pipe, feeding into what size pipe.

Give us this info and picture if possible and we can help.

By adding a compression fitting and then washer hose u could make a closed system.

Chuck

Vents do not repair backups. Chuck gave you the straight dope on what could cause your back up and a solution to fix it. If you have a 1 1/2" standpipe and drainage then Duct taping the hose into the stand pipe or installing a compression fitting,(see image) is the way to go. The reason for this is the today's washer pumps are much more powerful now. I had to make my own washer into a closed system because of backups. Good luck, Tom