Check the harmonic damper.
The HD is actually made of three parts - an inner steel hub, an elastomer ring (think "rubber band") and an outer ring.
Over time, the rubber band starts to wear and the outer ring can slip - or the rubber band starts to squidge out from between the two steel parts.
If the outer ring is starting to "walk," it will force a misalignment of the belt and it will try to walk off the pullys. The damper will carry the belt in the direction it is walking.
Since it's walking inwards (likely,) you don't want to waste a lot of time on this - there have been cases where the HD has walked backward right through the timing cover! This can be prevented - just change the thing if it's walking.
I'm assuming your previous belt wasn't doing this - so it just started walking.
The quick way to check is to lay a steel straightedge along the front of the damper, and see if either part doesn't touch the rule. That will tell you quickly what's going on.
5-90