Cooked burger meat Turning Brown from Pink instantly after being exposed to air?

The color change in meat has to do with the hemoglobin left in the tissue- even though it was bled during the butchering process. It will keep that nice pink if the meat was treated with carbon monoxide prior to purchase. That tends to keep it nice and red/pink until it is cooked. You didn't say how you cooked this meat, but I suspect your meat was treated and couldn't lose the gas until it was exposed to air. It's also entirely possible that Eric is correct, and you had carryover cooking take place.