That is very strange for northern Illinois in mid-November.
Well, I’d say that the many nights this fall when the plants weathered temperatures in the mid twenties Fahrenheit rules out climate change having anything to do with their longevity. The freezing temperatures burned off the flowers but the plants still look hearty. To my knowledge, petunias were never a frost tolerant planting so I did some research for clues as to why these plants were so durable and came up with a possibility.
In 2004, researchers came up with a way to make petunias tolerate temperatures as low as twenty degrees Fahrenheit by splicing in the gene of a cold-tolerant weed. The plight of gardeners was not their motivation; the growers that supply all the garden centers could save millions of dollars in pre-season energy costs.
Being as it is now eight years later, my guess is that this (or similar) research now has frost resistant petunias on the market and I have a crop of them hanging on until the bitter end.
(Photos below are from early July)