That depends on your airlines. In general, all Nepali citizen required visa even only for transit within airport in HongKong. Read http://www.immd.gov.hk/ehtml/hkvisas_4.htm#part1 

Although, I am sure that if you have Cathey Pacific Air from US to HongKong and afterwards if you have either Cathey or Dragon air, the airline will make special arrangement, you do not need any visa. I don't know about other airlines. You can check with your airlines. Even  if you have Cathey, you can call them and make sure.