If you have been riding with the same tires all year, they are getting thinner as they wear. Thus they are more prone to punctures. As well, the rubber in your tires may be drying out a bit and may not be as supple as it was at the start of the season. When I was commuting a lot, I used to notice that my tires became much more flat prone as they wore down. The rubber would dry out and become less flexible. Wet roads make things even worse. Small sharp rocks tend to stick to the tire tread and work their way through the tire casing. The thicker rubber in a new set of tires is more resistant