To follow on, there has already been reported autopilot drifting and mismanaging that spot on the freeway (although without this catastrophic result). That fact adds to this description/explanation.

Aside: the following another car heuristic is dumb. It's ultimately offloading the AI/decision making work onto another agent, an unknown agent. You could probably have a train of teslas following eachother following a dummy car that crashes into a wall and they'd all do it. A car 'leading' in front that drifts into a metal pole will become a stationary object and so undetectable.