1993 Bonneville rear brake lock up
 

I'm new here & this is my first post. So please bear with me!
My 1993 rear brake on passenger side has been seizing up every now and then, but would release pretty quickly. It's been making the sqeaky noise for a few months. When I backed out of my driveway today, it seized. But it didnt let go this time. I called my husband, who is a mechanic. He took the tire off, the drum had to be pulled off with a 3 jaw puller. To his amazement, everything on the inside looked new, even the brakes. We're stumped. Can anyone help?
Thanks

Have him check the lines etc. Maybe something hit or crimped the line to the other side and that one side is getting all the pressure.

1993 bonneville
 

Thank you.