White smoke out of a diesel is unburned fuel usually. The grid heater on my 98 dodge diesel hasn't worked in years, when I first start it in the cold weather it'll pour white smoke until it warms up to operating temperature.
It sounds like something caused it to overfuel while you were coasting against the exhaust brake. I don't know enough about the electronic common rail setup to know why it would do that in that situation, but it sounds like the computer gave it more fuel than it needed at the time.
