What happened to TYGER forces after Hugo Strange's death in Arkham City?
Most of the Guards had post-hypnotic suggestion, and it is implied that every Tyger Guard was either a hypnotized inmate or a mercenary who was mentally hypnotized to do strange's bidding. When Strange Died, they were like a chicken with its head cut off; they had no idea what to do, so they simply returned to base and awaited further order, however the GCPD came into Arkham city and rounded up the Tyger Guards and began dismantling Arkham City. In short, they had no more instructions, so returned to base/Arkham Island (if you unlocked that story) and sat around until GCPD regained control. In game, the helicopters de-spawn (to head back to base in theory).