The only instinct deer have to stay alive is to run. Just like a horse. That's all they can do, so if they're pointed in that direction when they're spooked, they'll just dart as quickly as possible.
Saying that, I don't know why the stupid cage drivers didn't slow down, too. Also, where there's one, there's usually more. If I see a deer anywhere near me (whether in the car or on the bike) I slow way down, because they try to stay "covered" as much as possible. Therefore they like to suddenly appear out of nowhere right in front of you.