What just happened ? Belt squeal and charging full?

Took the 70' Dart to the auto parts store for a new battery. The old battery was just getting weak and it was 8 years old. Ok, no problem. Drove it to the auto parts store. Got the battery in and drove it home. I was almost home and heard the belt squeal like something was locked up. I looked down and saw it charging like a mother. I almost shut the car off, but the squeal stopped and it came back to normal or slight charging. That alternator isn't more than a year old and probably only 2 hours of time on it. What the heck happened? Why did the alternator "brake" and then free up? Voltage reg issue? A few years ago coming home the headlights got super bright for a second or two. I was almost in the driveway then too. I think I changed the regulator after that. What just happened ? Thanks.