I can understand how the price could be manipulated by buying a shitload of BTC in the Non/Dec 17 run up and then causing panic by dumping it all.
Simple. You make legal bitcoin businesses pay as much taxes for 2017 as possible! Dec pump then Jan, Feb and March dump to make them sell as much bitcoins as possible during Q1 to meet their 2017 tax obligations. Expect we are very much near the bottom for the entire 2018 right now.