For belts to stay on a mower where the belt run is not in a single plane all of the pulley spindles must be parallel to each other.
Usually the tensioning arm wears and allows the tension idler to tilt
Once the tilt gets too much nothing will keep the belt on.
People rarely lubricate the pivot on the tensioning arm.
SO the hole flogs out oval then the arm itself can twist and the twisting action wears a groove in the underside of the arm which allows more tilt in the pulley spindle .
This is a self energizing set up.
I weld about 30 or so tensioning arms up every season, making the hole round again and the bottom flat again so it can only swing in an arc which is true to the axis of the pivot bolt.
On those JD decks the spindle housings are quite strong so rather than breaking like the Husquvarna ones do , shock loads tend to twist the deck pressings so the pulleys on the spindles are no longer co-plainer .
Of all the mower I repair, the JD decks require the most 10lb hammer rectification.