Both of my cars turn on high beams automatically on dark roads when they can't see other vehicles

The not as good at it as me though, the cars don't see the headlights of an oncoming car illuminating the side of the road, they need to see the headlight directly

Likely just the increasing brightness of the bulbs. Lots of times I will assume someone has high beams on only to see them flick even brighter once I go past them.