The red box runway markings are actually warning you that you're approaching the runway 09-27 and are on the taxiway side of the taxiway/runway demarcation line (the one that's two solid lines and two dashed lines). You can also tell which side is which by the demarcation line, since the dashed lines are on the runway side and the solid lines are on the taxiway side. So technically the bomb actually exploded on the runway, although on a portion of the "runway" where aircraft would be transitioning to or from a taxiway and traveling slowly as you said. Also it's very close to the edge on a part aircraft shouldn't normally roll directly over with their tires, but it certainly could have been under a wing.