Why is there water coming up under the toilet when you do a load of laundry?

When a washing machine drains itself of water, it usually does so very rapidly, pumping out the water with considerable pressure. If there is a partial blockage of some kind in the sewage pipes that serve your house, it might not be severe enough to be noticeable under other circumstances, but the pipes get overloaded and back up when the flow of water is too great.