All the requirements and are available at the Consular section page of the US Embassy site in Jakarta:
First, the consular official who conducts the interview comes away suspicious that the applicant does not plan to return to Indonesia. Applicants can mitigate the chance of this outcome by documenting strong ties to Indonesia -- Job/income, family, property ownership, money in the bank, etc.
Second, the applicant's name is similar to one on the State Department's or Homeland Security Department's watch list, or the applicant is from a region know for producing or harboring Islamic radicals, or there is some other innocuous bit of information on the application or gleaned during the interview that raises a suspicion, reasonable or not, that the applicant could be a security threat. I don't know that there is any way to mitigate this basis for a denial. I think its rather arbitrarily applied, even more so than the first reason.
Stay cool and don't let the hassle get to you. Good luck with your sponsorship.