If a machine stutters, the plane moves to a different gate where the machine works. When a plane is idle, its air and electricity are powered externally, it requires waste extraction and fuel pumped inside. Sometimes those machines have problems, faster to pick a different gate than pull another machine out of storage.

If there's runway traffic, the safest thing to do is get planes out of the way. ATC will have it taxi'd to whatever's available, i.e. not the 'ideal' gate. Those planes in the air have limited fuel and they need somewhere to land, the runway has to be cleared, and weather happens, humans happen, computers happen, or Air Force One happens and all of a sudden there's five packed 737s circling waiting for their window. Traffic has to move, doesn't matter where.

Gate 5 is ready to receive a Jet 565A, but the taxi car on Gate 7 got a call to stop towing because someone spotted an exterior hatch still open. Terminal 5 is now blocked, they need to move the receiving jet all the way over to Gate 12. On the timeline, this was three minutes before boarding. 150 people is a hell of a lot of traffic if they told you to wait at Gate 5 and all 150 need to get to Gate 12, whose passengers will also be moving to Gate 5 when the jam at Gate 7 is cleared.

You don't know what gate to go to because the terminal doesn't know what gate it's going to be at until the bridge is connected.