From the Legends perspective, I see it as backlash from events depicted in *Rebel Dawn*, the depiction of Bria's (Han's love interest) actions towards Lando, confiscating his property in the name of the Rebel Alliance.  Having lost face and being branded a traitor back on Nar Shaddaa, Lando refuses to believe Han was not involved in the swindle, and punches his former friend in the jaw.