There is three things that can make the Trangs hostile1 : You are working for Trang AND Umpani and your cover is blown, it usualy happens if you got the flag mission and gets enlisted by Umpani BEFORE you buy the flag
2 : Fighting a Trang or group of Trang will make them hostile
3 : Entering resticted ares, you will be told its resticted, maybe the teleporter (I havent used it, so I dont know) is in a restricted area and its possible that Trang wasent quick enough to attack you then you entered the teleporter, that will happend if you enter a restriced area