What would cause the serpentine belt to come off of my 2001 chevy tracker
Answer: There are several reasons why the serpentine belt could come off your vehicle, or any other vehicle. The most common reason is that the serpentine belt is just worn out and needs to be replaced.
Another common cause is that the belt tensioner is froze / stuck and not functioning. The tensioner does exactly what it's name says- keeps tension on the belt. Without tension the belt will come right off. This is the part that you must adjust or release the tension to install the belt.
Another possible cause is that one of the components with a pulley that the belt rides on is worn out and not sitting perfectly straight. This could be the water pump, power steering pump, A/C compressor or what is called the idler pulley. The idler is different than the tensioner. It is a solid pulley that just keeps the belt riding true. What happens is that the bearing for any of these pulleys the belt rides on is worn out, so the pulley either wobbles or does not sit flat and gives the belt a chance to "walk" off the pulley. A worn out pulley will usually be accompanied by a growl or whining noise when the engine is running. You may also see that the belt is worn out on one edge.