Many households use dishwashers without any significant problems. But if you've noticed that water seems to be pooling in your dishwasher, or worse yet, overflowing to the point where it pours out onto the floor when you open the door, then you likely have a problem with your plumbing. Here's what's causing water to come out of your dishwasher drain when you run the sink.
Understand What's Happening
First of all, it's important to understand that your kitchen sink and dishwasher most likely share a single pipe that carries away any wastewater. They are both connected together in this way.
You might not think that you have a problem with your plumbing if your dishwasher is able to drain and the sink drains as well, but that may be just masking a problem. You're likely experiencing a partial blockage deep down in the pipe.
The reason why you're having water come up in your dishwasher is that there's nowhere else for the water to go. When your dishwasher drains, it pushes a large amount of water down with a huge amount of pressure behind it, which likely pushes the water past the partial blockage. But when you run your sink, there's far less pressure behind the water. As a result, when the water (or waste, if you have a garbage disposal) encounters the blockage, there's nowhere for it to go. As a result, it'll come back up, but it will often go to the dishwasher instead of the sink as the dishwasher is closer to the ground and less impacted by gravity.
What to Do About It
In this situation, it's important for you to contact a plumber. The blockage is likely beyond the sink and dishwasher itself, which makes it difficult to treat with something like a chemical drain cleaner. To make matters worse, there's a chance that if you use a chemical drain cleaner that it could end up coming up in your dishwasher, which could damage it.
Instead, call a plumber who will bring non-toxic and safe methods to clear this blockage. One example method is hydro jetting. Like the dishwasher's pressure, hydro jetting uses a great deal of pressure to blast away blockages and encourage them to go fully down the pipe all the way to the sewer line. This will break up the blockage and should immediately stop the process of water coming back up in your dishwasher.