Don't recall a clause in the constitution but what started it off was at the height of the Swales campaign Frank Horrocks was in the MEN claiming the the OSC had 100% support for Swales.
I had a letter in the MEN published, stating that as a member I wasn't consulted and neither was the branch I was in consulted about whether or not I supported Swales.
At the next OSC committee meeting at the social club a vote of no confidence in Swales was asked to be taken and susequently refused.
Eleven branches resigned on the spot and the Independent Supporters Club was formed and backed Franny Lee.
I think the leading branches were Chadderton, Yorkshire and Reddish.
@yorkblue may be able to add to it.
I wrote about it in the fanzine Blue Print at the time, with what was my favourite headline to anything i wrote "The little (closed) shop of Horrocks"