When taming, there is a red bar where the taming effectiveness is. It will build up as you are riding the Andrewsarchus and, when full, it will kick you off and try to kill you. If you manually dismount before the bar is full you can avoid them aggroing and trying to kill you. Wait a few seconds before trying to mount them again to let the bar lower.

More Andrewsarchus Taming & KO Tips