The Laundry/Mud Room Has Diarrhea

So the past two times I've washed some laundry, the second of which was Tuesday night, the floor has flooded.  Not bad; in fact, I don't know if this usually happens in houses.  But I went down to check if the washing machine was done and, for the second time in a row, there was a fairly large but extremely shallow puddle that I didn't want to step in.

The strainer in the basin, through which water flows underneath and, apparently, up through the grate on the floor in this room, is completely clean.  When it has been stuffed up with gunk, the back-up of water coming out of the washing machine drain hose (I think most houses connect that hose into the house's drain pipe directly, but we just nose that hose into the sink in that room because we're poor-thinking like that) makes the water fill up in the basin.  Sometimes when I go down there I freak out because the water is about to spill over the huge sink, so I'm grabbing cups to bail it out of that side of the basin and into the other side.  Maybe that's why I'm freaking out over seeing the pool of water on the floor; for that not to happen, the water would have to fill the basin, and I'd be having a heart attack over that instead.

Come to think of it, why can't the water just run into the sink, through the strainer, then underneath the grate and out?  In other words, why can't it just be normal?  Why do I have to worry about either flooding the mud room or the water overtopping the basin?  But because of this clean strainer and, I guess, something backed up underneath the floor, the entire room has, like, diarrhea or something.

Whatever, man.  Unless it starts to truly flood, I'm not going to care about water coming out of the floor.  Yet.