A couple of years ago I was in the process of greasing my axle bearings and found what you did: the brake on one wheel was falling apart. In my case, it was the brake adjuster and related springs that were rolling around the wheel housing. I was able to just replace the badly damaged adjuster mechanism with no other apparent damage.
I suspect my problem began with a broken spring on the adjuster.
This can happen at any time. The only solution I know of is to be cognizant of how the rig is running and to take apart and inspect all wheels ever so often. How often depends on how much energy or money you have and how lucky you feel.