Since you had the head off, I'm thinking you left off the spacer washer under the camshaft sprocket when you put it together. It's easy to do (ask me how I know this) and will allow the sprocket too close to the head and the timing belt will walk forward. Although when I did it it didn't walk off so far it rubbed against the plastic cover.

The belt will ride out with a loose tensioner...the tensioner only squares up with the nut tightened, so it could also be a cocked tensioner.

Replace the tensioner at 150K or even every second timing belt change. Cheap insurance.

The small metal flange behind the crank sprocket is supposed to look convex with the edges back. If it is installed incorrectly, the timing belt will be shredded. Check the 700/900 FAQ via the 'features' pull down menu.

It's more likely that the tensioner isn't seated quite right. The peg that slides into the head has a habit of binding up a little bit when you insert it. You can still torque down the tensioner itself, but what ends up happening is that the tensioner is slightly crooked or askew, and sends the belt to the outside of the pulleys. Not that it's common, but I've seen it happen (read: I've HAD it happen).

I would pull the tensioner out again, grease the little hinge peg on it, and put it back in against the head. Retorque the tensioner bolt too. If that's not right, you can end up with just the same situation.