Maybe it started with Vader trying to find the pilot of that hunk of junk that not only spent a few hours in the hangar of the Death Star, but also later came back and shot him and gave the X-wing (with the pilot that he felt was strong in the Force) an assist? Vader figured that the ship was a smuggling ship and that maybe Boba might know something about it. Yada yada, he learns that a kid named Skywalker was picked up on Tattooine by that same ship. I mean, there was that Elephant faced dude working as an Imperial informant on Mos Eisley that sent Stormtroopers after the droids being smuggled onto that very same ship…