The driver was on I-80. Unless this is a Tesla employee, Tesla FSD is not enabled on interstate highways at this time; when you're on a major no-stops no-traffic-light highway, it defaults to Autosteer+TACC, with his subscription likely only enabling the auto-lane-change feature. All this means is that the likely culprits are either TACC stopping on its own as if there was a car there, or the AEB system performing an emergency stop to avoid hitting a (phantom) person or car; but based on the footage, it looks like a TACC failure since it doesn't look like an emergency stop you'd see in IIHS videos[0].

0: https://youtu.be/liQxaeGPHJg