Bruce and Harley knock the spa receptionist unconscious and the team enters the facility to steal the virus but encounters heavy resistance. In one of the attacks only Bruce manages to escape. The achievement appears just as he throws himself out of the trap, leaving everyone behind.
Note: Telltale games often have problems with the achievements. To be safe always exit the game completely. Never use the Quick Resume option. Apparently it's necessary for the game to connect to the servers for everything to work well.