Cage driver is someone who gets in the car, switches on the radio, uses the heater and bimbles along, looking ahead only as far as the vehicle in front. They know they're perfectly safe with their roll bars, their airbags, their crumple zones, their ABS.... They're in a safety cage.
Consequently, the motherfuckers think nothing of bumping you out of their way and assume you will get out of their way because they're safer in their cage than you. This attitude may or may not be a consciously enforced approach, but the motives are the same.
Basically, the mass of idiots who don't want to be there, stuck in traffic and fucked off.
Those who don't realise or care that they're driving a weapon.
Typically, it's fecktard mothers in mini-4x4s and anyone else who has ever uttered the words, "Well, I'm safe".