cbt,
apshalt or blacktop? stuff gets pretty hot, with the bees 'parked' on it i would have to agree with what lee said:
"The question in my mind is how hot the surface of the tennis court gets at the end of a long and hot day. Humidity is a big factor as well, as you know, honey bees cool their hive with evaporative cooling. The concrete in my driveway can get well over 130 degrees with partial shade. Asphalt can get MUCH hotter! Brood in hives at those temperatures could be baked on those hot days. A honey bee's life or death limit is approximately 118-119 degrees F."
good luck and keep us posted as to what you guys figured out what might be the problem.