ask your company if they receive any fax from uscis? it could be rfe in case of rfe, uscis will fax the petitioner and mail out a paper copy of the rfe.
I think it is either an RFE(request for evidence), or because of security background check. Check this http://www.murthy.com/news/UDdelsc.html which says: Premium processing cases for employment-based non-immigrants will continue to be adjudicated within 15 days. However, if the security check results in a "hit" (i.e. an indication that further investigation is needed), the case will be transferred for investigation and the 15-day "clock" will stop, pending the investigation.