I'm not sure the volume actually did increase so much in November and December.
The actual amounts transacted each month were:
Jan 31 2011 3,679,198.19243835
Feb 28 2011 4,533,818.22264172
Mar 31 2011 9,331,953.39257620
Apr 30 2011 13,893,407.03577082
May 31 2011 11,850,890.09316489
Jun 30 2011 73,031,161.99932121 <-- biggest
Jul 31 2011 37,718,094.01889341
Aug 31 2011 17,348,635.19178118
Sep 30 2011 17,616,839.59721035
Oct 31 2011 12,008,040.32522139
Nov 30 2011 49,968,992.05725216
Dec 31 2011 46,978,824.26111485
Jan 31 2012 11,664,075.33329784
The blockchain.info graph appears to ignore coins which are moved more than once in the same block.
To see that the blockchain.info graph is ignoring some volume, consider block 133233. That's just one of the >100 blocks from 25th June, and it alone contains transactions worth 2,420,580 BTC. However, the blockchain.info graph shows that no single day in June had a volume over 1 million BTC. Scrolling to the end of block 133233 shows a whole page of small payments from a 46k BTC address, which the blockchain.info graph ignores.
According to the blockchain.info graph, the biggest day in December involved transactions worth 5.834 million BTC. There was only one day in December with that much volume, and that was December 6th, with 6.903 million BTC. On December 6th, the volume was dominated by a 440k coin being moved 15 times, accounting for 6.601 million of the day's 6.903 million volume, so the blockchain.info graph must be taking 13 of these 15 movements into consideration. The 15 movements occurred in 13 blocks - the coins move twice in each of blocks 156242 and 156346.
The origin of this huge coin was on November 16th, ten 50k coins were combined into a single 500k coin. The coin moved around a lot from address to address until 8th December 2011, when it was broken up into hundreds of small pieces.
Edit: see this graph of "bitcoin days destroyed", which ignores rapid repeat coin movements by weighting transactions by the amount of time since the inputs were created. It mirrors the data I pasted above, showing that June was a bigger month than November.