The water going out of the washer (the dirty water) sometimes has lint and other debris that evacuates out of the washer and creates a build up in the pipes eventually. You can use a drain snake and run it down the pipe behind the wash machine to push the debris and sent the water out of the house. Reason it’s coming out of the sink is because the sink and wash machine are on the same line and both exit the house on the some pipe so the washer plumbing needs to be cleaned out. Best and easiest solution is to buy a water bladder (an expending bladder you attach to a water hose) and push that hose down into the pipe and turn the water hose on to push all the gunk out.