When you drive outside of the track limits, your car noticeably slows. This is also true for your opponent. However sometimes, due to latency in either your connection or your opponent's, this appears not to be the case. This is because of an issue known as "Rubber Banding" where the position of other players can be inaccurately displayed on your device.
If you see, therefore, another player driving off track, this does not indicate that they are actually doing so.