Why do my muffin tops lean? I'm getting close to (my idea of) muffin heaven with this recipe which after trying a few others I've found to work consistently and well.

One small thing I don't understand is why my muffin tops, which are rising nicely to a delicious crust, are "leaning" or I should say pointing in a direction other than straight up.

It's not a problem since the muffins have good texture, taste, and a home-made look I find appealing, but I cannot explain why they lean.

Any ideas?
Perhaps I'm stating the obvious, but is your oven rack perfectly level? Muffin mix initially has low enough viscosity that it can be influenced by gravity as it gradually rises. Even a small inclination of the tray/rack is likely to cause this.