From my faulty memory: Tantive IV was supposed to transfer the plans (& VIPs onboard?) to a larger Rebel vessel, but everything went FUBAR when ISD Devastator hypered into the area. Speaking of party crashing, eh? The transfer was interrupted. Leia's ship escaped, but the Star Destroyer followed her... probably in a way it caught the Falcon: Vader was onboard and out of the possible destinations calculated from the jump direction he chose one: "Tatooine. That is the system. Set your course to Tatooine."
The Rebels probably didn't expect to be found so quickly: Devastator appeared right after they jumped in and opened fire. You know the rest
(Please note that games often mess up with official storyline)
Of course this is just a movie and movie storylines rarely make sense (like talking to your enemy for 10 minutes, thus giving him a chance for a rescue instead of promptly killing him etc.).