ttani,
As per what we have seen an interview can happen if,
a) You are a criminal
b) You have been associated with terrorist/blacklisted organizations
c) You have a misdemeanor
d) Your case has been transferred between centers ( I guess thats as bad as being a serial murder in INS eyes).
e) RFE wasn't replied properly
f) IIO didn't get laid last night.
If you fall into any of the above .. dude ur screwed.