I would attribute it, like you said, to an improperly tensioned belt or a failing tensioner that's just getting weak. It could also be someone didn't tighten down the tensioner bolt enough and it worked it's way loose a bit or something of that sort, aged belt, stretched belt, etc.
I'm in the same boat as you pretty much. I've been spending quite a bit a week trying to get my car back up to tip top shape and I'd really hate it if mine jumped a tooth or two or three and messed up my valves
especially since I just had a motor put in it. I'm thinking I'm good for awhile, so most of what I'm doing now are just little minor things like weather stripping or missing plastic pieces, renewing black parts on the outside of the car (like the black around the bottom of the back of the back glass)