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.