I'm surprised it's even a question. Obviously, Obi Wan's malfunctioning lightsaber was resonating with the disturbance of the force caused by the destruction of Alderaan, which clashed with Han's emotional distrurbance at cold heartedly murdering a green bounty hunter who obviously meant him no harm. This anomally was amplified by the Carbon freezing. Luke just happened to raise his hand to say "hi" and wipe some yuck off the nice guard's face during a spike in the anomaly which caused the guard to stumble backwards.

At least that's the story on StarWars.com