Frank - I realize that, but the way the OP has described it, the dragon is moving so quickly away from them that it likely did not land at all. If it did land, the PCs would get their chances to contain it via spells (Wall of Force, Web, Stinking Cloud, Ray of Exhaustion, Holy Smite, etc) and also the melee-oriented folks could charge the dragon as well and get in melee whacks (most likely, suffering only one AoO in total because dragons normally have average dex at best)
If it is flying and strafing them with its breath weapon, it will still have to continue moving forward towards the party in order to keep flying, leaving it in spell range for at least 1 round. If it is hovering & breathing, it will also still be within range of at least spells for a round as well.
If it strafes them at the limit of its breath weapon range (meaning, after a move action) it will still be within spell range for the rest of the round, and the dragon will either have to continue flying over the party that next round, or pay serious movement penalties to turn around and flee (40 feet of movement needed to make a 180 degree turn of four 45 degree angles) and still likely be within spell range.