Hole issues

this issue is addressed at http://artisanbreadbaking.com/problems/

I'll past the section relevant to your post below:

This is also called a “flying crust.” Some very good professional bakeries turn out breads with this fault and get praised for it. Go figure.

The dough was allowed to rise too long in a very dry environment. This is the primary cause. Here’s what happens.

The dough rises properly during the first part of the rise.
When the rise goes on too long, the interior, what will be the crumb, begins to settle back a bit, since the dough is getting weak.
But since the crust has been allowed to dry out a bit, it can’t fall back with the interior, so it stays high and dry.
When the loaf is finally baked, the crumb doesn’t rise up to meet the crust, and the tunnel stays.
The final rise period was too short. This is a very rare occurrence, but it can happen. Just make sure you rise for the proper time and do the punch-in test.

There is another instance of this problem.  Sometimes, a loaf will enter the oven looking fine, but will collapse in the center during baking.  This is a tricky problem to solve, but review your procedures.  Here are some probable causes, which may occur together.

You used all purpose flour when the recipe called for bread flour. Because of the lower protein content, the yeast ran out of energy and couldn’t give the final burst of activity to sustain the crust.
A wet dough. A wet dough has a crumb that is more like a lattice work than a solid. As the yeast develops, the lattice forms. When the bread is baked, the lattice collapses in the center because the yeast activity can’t support it.
A dough that has risen too long in a moist environment. This is a tunnel crust without the dry crust. In this case, the crust follows the crumb down.
Too much yeast for the protein level of the flour. The yeast runs out of nutrients and the dough has no energy left.

These are some possible causes, either alone or in combination. Because there are so many different causes and they can work together to foil your best efforts, the answer will probably be a combination of a few minor changes to the recipe. When you get it right, remember to note what you did to solve the problem so you don’t have to go through the whole process again."

In the bulk ferment, I would go longer. I say this because of the tightness you get in the bottom of the loaf. I usually go for about 4-5 hours or until the dough is airy enough for me to cut a small piece off the corner and see if it floats in water. I would also add more sets of stretch and folds, cause it might be that your dough is not strong enough to hold the bubbles as well. 

This loaf was underproofed, especially during the bulk stage. It could just be a matter of needing a longer or warmer bulk fermentation, but often times -- especially with newer sourdough bakers -- it's a matter of using a starter that's insufficiently active. An active starter is one that can at least double in volume within 8 hours of refreshment at room temp (though tripling in volume is better). That assumes a typical 100% hydration white or white/whole wheat blend starter fed at a standard feed ratio of 1:2:2 or 1:3:3 (parts stater:water:flour; all by weight). If your starter is incapable of rising that quickly then that's most likely your problem and you'll need to adjust your maintenance routine to bring it to full health and activity. 

Trevor