It may be that one of your 'open' accessories (e.g remote controls, underground sensors, intercoms etc) are causing it. Try to remove all of them an see if it still occurs. Also, make sure that the gate actually opens and not stay open after the last vehicle entry. In that case, the safety devices may cause it to stay open (photoeye, loops etc). If all else fails you can always contact Liftmaster for diagnosis to make sure there's no equipment failures.