Something worth checking when you're overhauling your brakes. I have now come accross this from two different bikes (Steamers) on calipers in different positions. The caliper plate was found bent approximately 2mm radiused between the two chassis mounting bolts.
I can't say which of the two likely protagonists caused this, uneven piston force or more likely one or both sliding pin / guides being at least partially seized. If not spotted you'll find it harder to get the mounting bolt threads started, the sliders will not be parallel and obviously it will put more strain on the alloy mounting lugs than is necessary.
It can be straightened held in a vice with protected jaws and judicious use of a hammered block of wood, my preferrrence is to use a rubber coated dead blow (lead shot) hammer as it gives a bit more control.