if you read back to last night - the thought was that they were planning to mix in with the exiting crowd to set them off. this morning, I am seeing this idea that maybe these buses outside the stadium were a possible target.
Think of the shrapnel that would have been blasted into the crowd if buses exploded. I think the primary cause of death in bomb plasts is the force of the blast itself. However this usually kills those in the direct path within a certain distance. The the second cause is shrapnel. Since shrapnel can spread over a much wider area you may actually have more fatalities from the shrapnel. In a bus you also have the fact fires would start. Depending on where the buses were parked I guess it would be possibe for the fire to spread. This is all speculation of course.