Rear Tires Suddenly Lock Up Going 65

Hmm..hopefully it's the brakes. As fun as a tranny rebuild sounds (I wanna do some motor work and paint over the winter anyways) I don't need another project. I don't understand how manually shifting plays into this. I can assure everyone that it did not accidently get thrown into reverse. I've heard rumors of it being bad on a tranny but never really understood why. Makes more heat, uses different bands and clutches..? I sometimes manually shift cause..well...it's funner that way and I can offer more control through turns or when passing. I don't see how the tranny could have taken this kind of abuse and still shift ok, manually and automatically.