That depends on what the security issue actually is and who is affected by it. Full disclosure
is attractive when I'm the one affected, but if the problem solely affects the Fedora Project
entity then they have a right to restrict information as they see fit.
However, it's pointless to speculate at this stage (and believe me I've been searching the
lists and bug-tickets for possible clues!) and may be something mundane involving running out
of resources for crucial infrastructure servers or completely broken packages being made
available for updates. It's hard to guess why such causes would not simply be stated as a
reason though.