Static State Theory states that vehicles that stop moving will be frozen in the position they stop moving at and at the location they stop moving at until another force acts on them. This allows for the creation of balances and explains Leaning Balances as well as Phantom Balances, which both involve vehicles being in contact with other objects such that their velocity and acceleration are rounded down to zero – putting them in a static state.
Note: The following is extrapolation with little to no evidence. It is all purely conceptual, and thus may be inaccurate.
It can be reasoned that Static State Theory is a simplification likely created to address certain larger more potent problems if gravity always acted on all objects. Given the nature of Halo: Combat Evolved's physics engine it is possible that, if vehicles could not revert to Static States, they would always be moving, at least slightly. Thus vehicles would always slide downhill, sort of like how a Multi-Vehicle-System slides downhill, if vehicles could not revert to a Static State.
It can also be reasoned that a Static State System would operate by rounding down velocities and accelerations below a certain preset value, say 0.001m/s or 0.001m/s2, to zero. This must be true because a game is not analog – it is digital – and thus needs to calculate finite numbers and decimals and can only calculate so many digits per value as can be properly computed and stored by the CPU and RAM of the computer running the game. This rounding function would be how a vehicle would revert to a static state – which has been specified to exclude the affects of gravity upon the vehicle.
It can be reasoned that other anomalous occurrences might occur if vehicles were unable to revert to a Static State. Games other than Halo: Combat Evolved likely have a Static State System as well – though they likely have checks to prevent vehicles from floating in the air.