'Apparent' means clearly visible, understandable, or obvious. When something happens for 'no apparent reason', it means there seems to be no clear or obvious explanation for it - the cause is not visible or understandable, even if there might be an underlying cause. 'Apt' means suitable, 'amiss' means wrong, 'ample' means plentiful, and 'appalled' means shocked - none of which fit this particular usage.