Oh. You're one of "those people" that don't use your turn signals properly.

/Teasing
Took me a few minutes to figure it out. When you change lanes and don't use your turn signal, that's when it warns you. Said the other way, it warns you when you change lanes without signaling. This is exactly how it should work: stay outta my way when I'm changing lanes, and it will if you use your signals every time. How else can a computer possibly determine your intent to change lanes? Direct wire to the brain maybe. (Coming in 2028!)
It's giving you one chance before it takes over. And when you crash anyway, a message pops up "I tried to tell you..."