Active Track is done with the DJI Go app software. As such, it goes by what is on the device screen. You mention that the video was breaking up. This is a very good reason why subject tracking was lost. It is possible that it got "confused" just prior to that and thought that the subject was moving in another direction, perhaps toward the trees.
DJI's software for Active Track is _not_ very good.
DJI's software for Active Track is _not_ very good.