Dishwasher water backup
Check the drain hose
A dishwasher normally requires either an air gap or a high hose loop. Both are ways to make sure that water can get out of the dishwasher but can't easily get back in. In the case of an air gap, if it breaks you will have a mess. But in the case of a high hose loop, if it falls down (or was never installed correctly in the first place) then water can come into the dishwasher from using the sink.
A high hose loop is normally a flexible hose that goes from the dishwasher up to just underneath the counter, but at a minimum it must be above the bottom of the sink, and then goes down to either the disposal (most disposals have a special connection for dishwashers) or to the sink drain pipe just before the trap.
If you are not sure how your dishwasher hose is connected, post a picture.