I think you are correct and what you see it is the polyurethane coating chemically breaking down. Warm or damp conditions will accelerate this. An example image is displayed on the Outdoor Gear Lab tent-buying page under the section discussing polyurethane coatings.
Usually this means it's time to get a new tent, although there is one thing you can try. A thread on BPL mentioned the following steps as received from one tent manufacturer (Sierra Designs):
- Wash in front load washer or soak in a tub with a laundry detergent until the laminate peels off or is no longer sticky.
- Make sure you have all the soap out of the fabric and that it is dry.
- Re-laminate with Tent Sure by the McNett Company. This is a paint-on, and 2 coats should be sufficient.
- Make sure the tent or fly has time to dry between coats.
It is also mentioned that this is not guaranteed to work. However, many people use similar techniques (often with silicone coatings) to seam-seal their shelters, and I have not heard anything to suggest such coatings are particularly temporary.