Yesterday I baked Trevor Wilson's pan sandwich bread [1]. Details of my bake are in my blog about this bake. [2]

My question is about the compression of the crumb along the bottom and sides of the loaf. This doesn't happen to me all the time, and I don't understand what causes it when it does. Note, this photo makes it look gummy/underbaked, but it isn't at all in real life. It's a bit of a lighting quirk in the photo, but I realized that made it easier to see the compression of the crumb in the pic. 

What is the most frequent culprit: is it a fermentation issue? Hydration? Not enough gluten development?  And don't get me wrong, the loaf is more than edible - it's quite pleasant and already 2/3 gone, lol. Any info is appreciated.

Thanks all!

Mary