The "-and" or "-end" suffix comes from the Latin gerundive, as I understand it, and generally implies an action that is, or should be, or will be, or might be, done to the thing being described. A dividend is something being divided by a divisor; a subtrahend is the thing being taken away; an ordinand is someone going to be ordained; a reverend is someone being revered.

Now you mention it, "graduand" is a bit anomalous there, because we typically say that one is graduating rather than being graduated. Perhaps the passive form was in use at some earlier time (and/or in Latin) and the gerundive form of that usage has survived even though the direct verbal form has fallen into disuse.