My Tesla doesn't change lanes without driver confirmation. It requires ACTIVE confirmation from the driver, and is extremely conservative in crossing the line. I frequently take over when it doesn't change lanes fast enough.
Most likely the driver was passing someone.
This discussion is under a nonsense test report and headline btw. The car was moving slowly, around what could easily be identified by gps and vision as some sort of test track or empty lot, with varied and inconsistent markings and no other traffic in sight.
The software running on drivers has been under active development for much longer than Tesla. (I'd start bidding at billions of years, though I'm willing to get argued down to tens of thousands.)
The lane changes are so slow that you have plenty of time to correct if the car makes a mistake.
You're supposed to have your hands on the wheel, and pay full attention, at all times. This means being able to take over if the car drifts into the oncoming lane, or has trouble deciding what the lane is.
That's why I call BS.