I can give you a definitive answer because I was the one who sent the UAL around....
The B777 that landed in front rolled past the A10E turn off and due to A10W being closed to work in progress the next available exit was A11, which is quite a bit further down the runway. No drama or anything, so sent the UAL around and seconds later it was all resolved.
HD is indeed correct in that at that time of the day it is very busy, particularly with heavy, long haul types that have been in the air for very long periods of time.
Hope that explains your question