In almost 5 years of making sourdough, I've only gotten an ear once before. Turns out my starter sucked. When it died (broke the jar, got glass in it) I spent 6 weeks trying and failing to make a new one. I still don't know why it wouldn't work, but I ordered one from King Arthur, and it's healthy and functional.
I was worried about fermenting this lemon rosemary sourdough since the directions basically only do it for an hour. I wasn't ready to bake yet, so I stuck it in the fridge for a couple hours. It came out beautiful and delicious.
It's the form of the crust. It only happens when the oven rise (the amount the bread grows in the oven) is good, which needs quite a few things going right, from amount of fermentation, to temperature, to good gluten network development etc.
Thank you!