I haven't planted burfords in some time, but it could be too much water with the recent rains. Is there good drainage around your shrubs (decent slope with non clay soil)? How much rain have you had and how much are you watering?
Plants also sometimes drop leaves in the hottest parts of summer to better survive. If that's what's happening, it'll be fine as long as it gets regular water.
Dropping green leaves seems weird to me, but I don't have enough experience with hollies.
Ha, yeah, but I think sometimes a problem can be identified if the leaf turns yellow or brown and then falls or if they're just falling off green. I just don't know what's what with hollies.