dkatwa
First, the terminology. Planes don't circle, the patterns are "racetrack"
Another thing that can result in holding is the weather. Down my way if a line of thunderstorms hits the airport, numerous things can occur. There may be a need to change runways and aircraft will be requesting vectors clear of cells. This makes it very busy in the terminal area so arriving aircraft outside the terminal area mal be placed in outer holding stacks. When thunderstorms are in the area, ground staff are not permitted out on the aprons (WHS requirement) so aircraft cannot park.
Similar to above, a fast moving front may require a runway change. A tower tries to stay ahead of the change but sometimes they get caught by the speed.
Rapidly deteriorating weather below the CAI I ILS minima can also cause problems. Greater spacing is required between approved CAT II and CAT III aircraft and if there are any CAT I one aircraft in the mix, they have to be put somewhere.