My 78 CB550K had some strange behavior with power changes. Turns out half the rollers in the roller bearing on the sprocket side were smashed, disintegrated, and gone. Most of the time the remaining rolers would space themselves out and the bike would track as predicted but radical power or descelleration (well, for a 550, anyway) and the rollers would bunch up and the wheel would skew out of alignment. Kind of like riding on marbles. Coasting and they would feel fine again. A new bearing solved the problem. But, I imagine if the brake disk side axle bearings went out in a similar way, it might feel as you describe under braking conditions.
Just a thought...
Cheers,
