Strange Oven Spring

Here's a strange little adventure I had today, with an ordinary white loaf. 

I proved it in my nice new proving basket, turned it out, and scored it with a serrated knife. Now, as far as I could tell, and I was careful about this, all the cuts went to the same depth.

Here's a shot, just after it went into the oven. The loaf looks jolly nicely shaped and even. The baking stone should have been evenly hot, but maybe it had not been heating up for long enough, and was hotter at the back of the oven. Quite early on, there was an obvious bulge at the back of the loaf. Yes, Deirdre, I do know the glass oven door could do with a clean.

Still, the loaf was rising nicely...

And, when I took it out of the oven, it was very obvious that the "oven spring" had not occurred evenly. One of the cuts has taken up almost all of the stretching that the loaf has done. I ask you all to theorise as to why it is like that.

• Uneven depth of scoring cuts?
• Unevenly heated stone?
• Shaping stresses in the loaf not evenly distributed?
• Fan oven hotter at the back side of the loaf?
• Act of God?
• Something to do with the curvature of space?