The PU layer on your tent fly is perishing.
Some think that it can be slowed down or prevented by letting the fly off-gass*, in other words not sealing it inside a tight stuff sack but leaving it open when in storage. The bad news is that it isn't practical to fix, the good news is that (apparently ) the new versions of the PU coatings don't have the same problem.
Or you can buy a silnylon or DCF (Cuben Fiber) tent , those fabrics/laminates don't suffer from it.
* you know that "new smell" from a car or matress/furniture ?
It's the material off-gassing.