It wasn't left in the sun here. As soon as it was signed for, it was set inside my air conditioned house. I opened it after an hour or so. When I first ran my hands over it, to check for any splinters or cracks in the finish, I did notice that it was warm to the touch. When I say warm, I mean that it wasn't at room temperature. Maybe a little more. The bow definitely wasn't hot enough at that time to cause me concern. It was only a little before noon, and the temp might have been around 90* or so outside.
The more I think about it, the more I think it probably was left in a hot warehouse or truck over the weekend. That is the most likely scenario. As easy as it broke, the glue must have been heated up enough to fail. I bet it didn't reach 25" draw, before it went.