It's hard to guess, but by now savvy investors will buy lots of gems between holidays and announcements and then dump them when the price spikes. I can see some of the big money gurus with hundreds of golds invest in gems during down times and dumping at that spike. Especially now that there are internet tools that will send you alerts when prices hit a point you set.
In that graph gems went over 2g to buy 100 or 1g60s to sell them. In the first weeks of November buy prices were hovering around 80s say you spent 100g buying gems you would get 12,500 gems. Then they spike to 1g60s sell price because of Christmas speculation (or in this case hair and appearance kits) and you sell all 12,500 gems you would walk away with 200g.
Its hard to tell what caused the drop because there is no volume of sales graphed there. If it was less than say 100k gems (800g in early November) I would say it was 1 or 2 people taking profits. If it was more than that I would say Anet had a hand in the drop.
