Andy, Never had that happen when its been sitting for that long of a time. But I have seen it happen when tires are overfilled with air pressure, and have had it happen to me while on a road trip, had been driving about 300 miles straight so pulled off for food and walked about 15' from the car when it blew.
Was there any signs of excessive wear on the tire before it blew that you noticed? I see the tire is a little worn, Can't tell if the seam split on the side is from the explosion or the cause of the explosion.
But I would have to guess that the tire was old and with it sitting for 4 months in the same spot the pressure just weakened the top so much if finally blew.