Why are sourdough crusts thicker?
Here's a scientific breakdown:

Sourdough starter is the primary leavening agent in any sourdough, including your hybrid here. Technically, it's just water and flour. The flour already has all the yeasts and bacterial spores it needs to ferment, and the water activates the fermentation process via an enzyme called amylase. 

The amylase breaks down the starch in the flour into sugars, which the yeast will metabolize, producing carbon dioxide gas. Anything in the sugars that the yeast can't eat will be fermented over time by the bacteria, producing more amylase and breaking down the sugars further so that the yeast can eat it. The bacteria creates lactic acid as a byproduct of this process, giving sourdough its unique "sour" flavor.

However, the same process that breaks down the sugars in sourdough starter can also break down the proteins in the dough. This gives you weaker gluten - or, a denser, chewier bread than what you'd obtain with only baker's yeast. On top of that, your bread is a hybrid, which means there's even more yeast in it than usual and even more leavening. More leavening, more enzyme production by the bacteria, more potential for protein breakdown.