There is a flange on the alternator pulley, so when the belt rides up on it, the diameter of the pulley increases and effectively shortens the belt. The belt tensioner arm has to move to compensate for that. Once the belt drops back into the correct position, the tensioner arm snaps back to take up the slack.
I hope that makes sense!


There is a lot of lateral belt movement on the idler pulley, so it could possibly be a small misalignment of the tensioner assembly, but I'd go with Ekskaar's suggestion first and examine the idler and its mounting.
Check the mating surfaces are clean and give the pulleys a spin to ensure they're running free.

Alternator bracket is also a possiblility, but IMO not the problem here as the misalignment is happening before the belt reaches that pulley.