October was my first full month with the iPhone 5, so that explains that. With Apple's cloud service they allow you to basically download your entire old phone to your new phone without hooking up to iTunes. I did this and blew through my 5 gig limit without realizing my wifi wasn't turned on (I was at a hospital during the birth of my sons and the wifi was spotty).
I have unlimited data grandfathered through AT&T, but they throttled me down to about 1-2 MB/s for the remainder of the month. To be honest, what they throttled me to was about what I was getting on the 3G service (or what AT&T was calling 4G), so it's not terrible, just not nearly as good as LTE.
The increase in November and December (and part of September) relates to the fact that I don't use wifi at home anymore. LTE is simply faster than my home internet.