The water from both the kitchen sink and dishwasher flows into the same drainage line. A blockage in the main drainage line causes water to overflow into your kitchen sink.  Kitchen sink clogs typically occur in the air gap, dishwasher, garbage disposal or sink—all of which can cause your sink to fill with water when the dishwasher runs. To find out why your dishwasher is backing up into the sink, call the plumbing experts at Plumbing Duck Inc. We will identify the underlying issue and fix the problem fast.