Left keg in hot garage, now it is sour? (but sour in a good way)

I brewed a 10-gallon batch of pilsner in June. We drank all but half a kegs worth in August. I rotated the beers on tap and set aside the keg to let if lager a little bit more. Well, I ended up needing the space in my fermentation/lager chamber and set the pilz keg next to the cooler in the garage. I meant to bring the keg inside, but forgot. I didn’t realize my mistake until a week later and just figured that any damage from the heat was already done. It sat in the hot garage for at least 6 weeks until a week ago, when I had enough room to cool it back down and I moved it to my kegerator.

The beer: It tastes fine, but I am noticing a different mouth-feel and a slight sourness. I think the beer tastes wonderful, but it doesn’t taste like a pilsner anymore, it now tastes more like a Belgian.

I don’t know enough about lacto/brett, but would letting the beer warm up, speed up any lacto/brett growth and result in the new flavors? Or do you think the change in flavor is simply a result of the hot temps?

Unfortunately I cannot answer your question. I had something similar happen to a summer wheat beer I had kegged. I kept the keg in question inside sitting on a central air vent with a thermometer on top. The keg never got above 74F all summer. When the first keg of summer wheat blew (it had been refrigerated the whole time) I put the next one on and discovered it had a noticeably sour taste to it......almost like a mild lambic. I love it! I figured the change was so pronounced because of the high yeast content.

I have six kegs on tap including a winter ale from last November. I find myself drinking the winter ale slower and slower because it gets better with each glassful! The change among different beers varies greatly but they always taste better.
Cheers,
Bill