Fall off and Go downhill

Fall off verb - To turn away from a straight line or course.

Go downhill and fall off are semantically related in decrease topic. In some cases you can use "Go downhill" instead a verb "Fall off".

Fall off and go downhill are semantically related in decline topic. You can use "Fall off" instead a verb phrase "Go downhill", if it concerns topics such as lessen, diminish, decay.