Usher the parents out ASAP - I had one little guy do that out of the blue as well to me. What I found was if either daddy or I said, daddy will be picking you up after nap. Remember, you'll see daddy after you wake up from your nap. It works wonders! And I have the TV on for him at drop-off, seems to work.
Also works if my hubby is home, but that will change come Sept, cause he'll be back at work.

Just leave him be, and he'll come around. I just grab the kid from the parents (away from the door), and let them cry until they're ready to join their friends.