A fiber wad is far from being an incompressible solid.
An online acceleration calculator shows that an object accelerated from a standing start to 1150 fps in 3 milliseconds experiences an average of nearly 12,000 G's. That's the average. Since a shotshell obtains 80% of it's final velocity in the first 10 inches or so, the initial acceleration would be much higher.
Look at a fired plastic wad and note the deformation the pellets undergo.
Fiber wads get squashed quite a bit, but have the ability to rebound as does a plastic wad.
Put a fiber wad stack of cards and felts in a vise and reef down on it a bit. That's what happens in a shell too.
Now take that away, or make it really short, and you've taken away quite a bit of area for gas expansion.
Pressure will increase.
