This usually happens when the belt is getting wore out or maybe because it was handled with an oil based lubricant. I would suggest to test the traction or get it replaced.
Some possible causes might include a stretched,worn or cracked belt. The pulleys might be misaligned. If there is an idler of some sort that might be positioned incorrectly.
The belt is probably old and stretched, you may need a new one. Also, it could just be you are letting your grass get too high before cutting it. The extra strain on the deck may cause the belt to slip.
