What went wrong with this bread?

You got close, it looks like you had good gluten and structure, I think there's a maybe more than one thing you can do to prevent this:

• This looks underproofed: You have bigger air holes on the top of the bread than the bottom, especially apparent on the bottom right of the bottom picture. A bit longer in the basket would have let the dough open up a bit more. It's possible you over rather than underproofed it, but my money's on under
• A large air bubble may have existed in the dough, before shaping it knocking it back a bit would be good
• You need to slash the top just before baking. Bakers use a special knife for this but you can use a carpet knife or a box cutter instead. A quick slash or two a couple of centimeters (about half an inch) deep will let the loaf open up, and large air bubbles on the top can't form because the air can escape. I do a cross cut in the center of the loaf, 1/3 to 1/2 of the diameter of the bread

One thing also, if you don't have steam in the oven the crust will form too quickly and you won't get oven spring, resulting in a tight loaf. Put a pan of boiling water in a few minutes before you put your loaf in to bake, then take it out after 20 minutes if it hasn't all boiled out. This has nothing to do with the big hole, it's just a tip.

This looks like "flying crust". See http://artisanbreadbaking.com/problems/. The cause is usually that your loaf was allowed to rise for too long in too dry an environment.

Make sure that you proof the loaf in a humid environment. I cover mine with a large clear plastic box, which does the trick.