John Deere GT275 throwing deck belt

It's been my experience with similar decks that the idler pulleys that the BACK of the belt is running on to check closely for any up down play and make sure the belt is running close to their center when in operation. If they have any up down play the belt will jump off because it's not dependent on a V groove to keep the belt in place.

When working on decks that have issues such as yours and I've removed the deck for repairs and not sure if I have corrected the problem quite often I leave the blades off or remove them while troubleshooting so as to feel safer while off the machine watching the belt alignment in the pulley when the PTO is engauged.

Usually the idlers, especially the ones that swivel have belt stays to keep the belt close to the flat backed pulley.

I cannot see the stack pulley????????
 
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a query online gave multiple causes. Worn bearings in the double stack pulley was most common. It says that pulley and bearing wears fastest because it has two belts tugging at it. If it wobbles or has any tilt that will set up a vibration that tosses the belt. The tensioner idler arm bushing can also be a culprit

Sometimes these things happen not because of one problem, but an accumulation of several issues. I would start by inspecting the spindles and each pulley for bearing wear. Also check the pivoting tension arm for tilt or bending over time. Bad bearings in the spindles the blades sit on can cause it too.
 
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