Me here,

Did you have proper documents from your college saying that you are graduating and that you wanted your parents from back home Nepal to attend your graduation ceremony???? Also, you are graduating in the middle of a semester, right now?? that's strange! that might be one of the reasons they got rejected. if you write the reason the consular rejected your parents, you might get some more suggestions.

in many cases, if the applicant fulfilles the doubt raised by the consular, they are granted visa in the second attempt. so, good luck to them and to you.