The large number of bitcoin days destroyed on 2013-12-21 can be seen mostly in block 276158, and on 2014-02-05 can be seen mostly in block 284349. In both cases, the coins were last moved around 2011-03-01, and the coins seem to be controlled by the same entity.
For example, if you start with the 45592.95 BTC in address 1Kq6sr5A9bM82u1vRTByjCvvixm5dxwQuV, you can see that they are split into outputs of less than 5000 BTC on 2011-03-01. Some of those outputs are redeemed on 2013-12-21 and others are redeemed on 2014-02-05. The same is true of the 150000 BTC in address 1LYJHSep7gAtvePMPVdsXvnKeNM7ifHkEK.
As far as I can tell, the coins were just moved to a different storage location, and there is no connection to recent market activity or the issues with Mt. Gox.
There is always the posibility of a Over-The-Counter selling. Most thinking people would prefer to sell their >50k coins at a discount to reduce the friction of trading on an exchange.