There may be an abundance of conventions/trade shows in October, but it's not clear that attendees would be renting cars (most conventions are held in large metro areas where cars are not necessary) and even less clear that they would be using award miles/points for a business trip. Perhaps they are combining business and leisure travel.

I took a look at techexpo.com (a month by month listing of high-tech industry conferences). September seems to be the champ for conferences! A rough analysis suggests that September has about 33% MORE conferences than October. Both August and November have about 33% fewer conferences than October (i.e. August and November have 50% fewer conferences than September). It is only one data point for a single class of industry, and on only web site, but it seems representative. Seems to me something else is driving high rental usage in October.