Sourdough

I'm back at the oven after eight months or so and, of course, I need some suggestions.  The Buffalo starter ("Bubbles") is just fine now after a bit of coaxing but yesterday's loaf has a crumb with a number of what I've heard referred to as "baker's bedrooms," holes wide and deep enough to accommodate my index finger with room to spare.

What leads to this?  What adjustments do I make to avoid it?  Handling?  Proofing time?  Hydration?  Baking temperature?   I'm using the Basic Sourdough Boule recipe but with 60g of whole wheat flour and 40g of rye flour.