Rear Tires Suddenly Lock Up Going 65

The drums came off easily but not too easily. Didn't have to adjust anything. I havnt touched the brakes since. Been trying to reproduce the lockup at slower speed but nothing has happened. I'm on leave so it'll be a while til I get back and do the brakes. Then the tranny comes out. The saga continues haha yes it is mind boggling.

I meant to study the 904 plumbing and was gonna see if it's even possible for such a thing to happen, even after a few seconds from the last upshift into third. If the kickdown band for second stays applied in third along with the front clutch for third..what happens??? Haha