I don't think it is so much about it being a "bomb". I'm gonna take a stab and say it may be related to the ability to take over the control system of the plane via a laptop/wifi [1]. It would make sense to some degree if the US knew that an airline had planes that were "unpatched".
[1]: http://observer.com/2016/02/midair-hack-shows-the-dangers-of...