During a burial today, I noticed the coffin was buried facing East.
Where does this Christian tradition come from?
It's not a purely Christian tradition. Other religions practice this as well. There's an article here on ehow.com that gives an overview of the origins of the practice. Some highlights:
Also note that it's not universal. We have a cemetery tied to a Church a few miles from here where the graves all point to the North, and several others I've visited have some facing one way, and others at right angles - those graves are laid out to maximize use of the land.
Some people believe that God moves east to west. At the second coming of Christ at the end of the tribulation He will come in from the East moving to the west. The idea was too bury the bodies facing east so that when the rapture happens they will already be facing toward His coming. This is more tradition than it is biblical but they did it for that reason.
On the other hand as a Jew you always approached God as at the tabernacle moving east to west. This was important as the pagans always worshiped facing east so in a way you should bury them facing west as not to face the pagan rising sun. How a Jew in the Old Testament or a Christian in the church age and how they are buried is not important..