With the Society, big "events" are thought to happen in the Spring or particularly the Fall. Those of us who were old enough to remember the 1975 thing recall that the 6,000 years of mans creation would end in early Fall (October) of 1975. It was even mentioned in the Aid book at that time.
I'm not a lawyer or talking specifics here, but corporations are required by law to have some type of meeting annually to fulfill the legal aspects. Many corporations just have their voting members do this via proxy. The last I heard (maybe 10 years ago) there were about 175 or so "shareholders" in the Pittsburgh corporation. They are typically the heavies in the organization. To be one of the voting members, it is considered a privilege since you are considered one of the few.
I would have to look up the articles of incorporation by the Pennsylvania corporation for the dates and requirements but it is no more than just fulfilling the annual requirements by the law.
Since October is such an important time in JW land, why not have the Annual Meeting tie into that time frame?
Rub a Dub