I want to duplicate my bread mistake!
I think adding the butter twice could maybe account for it. extra hydration, then collapse.

That seems like quite a bit of fat already. (just based on what I've done myself) 

If the dough is normally close to being over hydrated, when it gets too close to the stage where it collapses, a tiny bit more liquid can tip it over the edge. Maybe even an unusually humid day.

I have a funny feeling that butter doesn't interact with gluten the same way as water, but I still think it's the butter. 

Whatever it is, I've made that loaf myself more than once :)