Heres a little General IRC info, about Invite Only. Not pertaining to any particular channels, nut just for the publics general information...
When someone wants a channel only "members" or known users can join they make it Invite Only...
When a channel that is not normally Invite Only, is set to that mode, and you are wondering WTF, it was not Invite Only yesterday, it is going to be temporary. Channels set this mode so that new users can not come in, this is done for any number of reasons such as, flooding, an attempt of a channel take over, clening up of the channel management, etc....
So if a channel you know, goes Invite Only on ya, and it was not that way even 10 minutes ago, you can bet its a temporary percaution to get some things cleaned up, then the mode will be undone..