This is exactly what started happening to my '07 MacBook Pro after years of solid use. The parts started getting a little looser, the whole body is slightly bowed (it still closes though), etc. Jostling it a little in its bag is sometimes enough to put it into a wakened fright, and whether or not the fans kick in he's gonna be a pretty hot piece of metal by the time it's out of the bag. Since it started happening I just make sure to shut down before carrying.
