[zipper]

“Unusual route requested” means, with almost 100 percent certainty, a weather avoidance route different from the filed flight plan. It’s not “unusual” for airliners to request maneuvering off course, or even to propose a different routing to air traffic control when there is weather to be avoided along their route.

[HiTech RedNeck]

A freak downdraft perhaps? Knocked the plane out of the sky?

[BuckeyeTexan]

Good to know. Without any information about the weather, hijacking was my first guess.