Dehydration is pretty normal, but it's also a sign that you've burned a lot of glycogen.

The body holds about 2400 kCals or 600g of glycogen between the muscles and the liver. The body also retains 3 grams of water for every gram of glycogen, so 1800g (4 lbs) of water.

It's unlikely you're depleting all of that in a 2 hour ride since your energy comes from burning a combination of glycogen and fat, but with zero fueling, it's reasonable to assume maybe half your glycogen is depleted, along with 2 lbs of water.