A car will boost to a higher revolution in park if the idle is set improperly or there is a leak. This will also happen if the idle becomes stuck or the oxygen meter indicates it needs to adjust the oxygen-fuel mix.
A parked car should neither idle high nor consistently rev up when it is running and parked. When this happens, it is a strong indication that there is a problem in one or more of the components. The most common problem is an error in how the idle is set. The second most common cause is a leak in a vacuum hose. It can also be caused by a leaky or clogged PCV valve. The least common but most expensive issue to fix is a faulty brain box that is setting the oxygen mix to be to rich.