My Craftsman LT2000 keeps throwing deck belt. I have replaced the old belt but the new one only lasted approximately an hour before coming off. One of the idler pulleys looks very shiny and appears to be at an angle compared to the other idler. Is possibly the cause?

Possible... all pullies and idlers should be in a common plane... meaning horisontal and in line with each other, the sole exception being the engine pully with will have a degree of misalignment.

The fact that idle is shiney tells me there is a lot of belt slippage and its being at an angle can very easily push it towards beign thrown or at the very least cause premature belt wear.

First step I would take is find out why its and an agle and take steps to correct it, whatever might be needed.

Breaks on both pulleys. If they don't work belt gets all messed up