Water is a solvent and a pretty good one at that.

Your phone is mostly held together with adhesives.

The water will soak into and destroy those adhesives after a time, but not so fast that you have to worry about a momentary submersion.

As it soaks, water begins to make the adhesive fail. As more water gets between the glass and adhesive, it fails faster and faster.

Depth inscreases this because some of the waterresistance is also caused by air pressure inside the phone at points that need to be open like speaker grilles. The tiny holes help prevent water from making it in due to the air inside the phone. Past a point, the pressure around you compresses the air in the phone, displacing it and replacing the missing air with water. So yes, it is mostly down to pressure with depth.