One of two things happened:
1. ATL intended for QR to operate the 777 into the airport, QR changed it too late to the A380 (after all the international gates were already assigned to other flights) and as such they had to park and bus passengers to the terminal.
2. DL threw a hissy fit and used an A320 to use up ATL's only A380 capable gate. Everyone on QR suffers. Poor planning on ATL's part.
Since I don't work for or have any insider info at DL, QR, or ATL, I am going to guess #1 happened.