I think MJewell is right, if that deer is in the brush and something comes up on it, it would have a hard time getting up to get away in time. By laying in the field, it was able to see danger from a long ways away (you) and was able to get up and get going before you were able to close in on it.
"Knowing when to shoot and when to wait is one sign of maturity in a bowhunter. Be patient and pick your aim point carefully." ~ Randy Ulmer
"I suppose it's the way of hunters. We are very odd fellows." ~ Peter Hathaway Capstick