Some guys may be heading to an airport with a ground stop, or a an upstream controlling facility has a flow program in progress that restricts the number of aircraft inbound to a certain airport.
Lot's of things though. All depends. 10 aircraft in line, the three at the end of the line do not have any ground stops, or enroute delays for their destination . . . so they are able to back taxi and take off, leaving 7 aircraft in line. The front 3 each are waiting for a "wheels-up" time, from an upstream controlling facility that has either a ground stop, or enroute / terminal flow restrictions. The other 4 aircraft are eventually launched as they are heading to airports with no restrictions, leaving the original front 3 aircraft waiting for their wheels up time to launch.