Did the Empire contact Lando or did Lando contact the Empire? What was Lando's relationship with the Empire in specifics? Could Lando have really had not gotten his deal changed and why do you think he couldn't have expected that it would?
It was the infamous bounty hunter Boba Fett who tipped off the Empire to the Rebel presence on Bespin (See Tam Madreus' related article "Free Agents in Galactic Struggles, 2-5ABY" in the official Archives of the New Republic, War Years).