Why are people traditionally buried facing east? 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:


According to "Ethnicity and the American Cemetery," the feet of the deceased face east as well. This tradition is based on the belief that when Jesus returns, the departed will rise from the grave already facing his direction. 
According to Northumberland County Council, the tradition began when Pagans buried the dead so they would face the rising sun. 


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.