Several possibilities:
- Arrival is full and they need to space them out.
- Amazon arrival or departure. They have 25-ish flights a day in 737s and 767s - more spacing/time required.
- Mishap on the runway.
I went Thursday with a neighbor in his -9A. As we were approaching Fantasy of Flight (starting point on the arrival), ATC was sending planes back up there, holding for an Amazon arrival. Our timing turned out to be just right. When we were about five minutes from turning inbound, they started sending everyone in. Easy peasy, no delay for us.
Going out was easy, too. We were in the plane ready to start midway theiugh the final airshow performance of the afternoon. The moment the red flag went down, flightline volunteers gave us the start signal and got us moving.