whats the reason for this down-syndrome ?
I don't know but I think it is uncertainty regarding the movement of old coins that could be tied to MtGox. Here is the latest discussion:
http://www.reddit.com/r/Bitcoin/comments/1zvx7i/coinsight_now_shows_another_160000_btc_transaction/
There is a "special page" on coinsight regarding Mtgox BTC transactions with some note : "Mt.Gox reports at least the following types of transactions through this API: withdrawals, dust sweeping, big output splitting. Some transactions have been spotted on this API recently with multiple successful block confirmations, which could just be a result of Mt.Gox wallet software not seeing the confirmations or could be new undocumented API behavior." :
http://coinsight.org/mtgox/largest
It looks like we also have blockchain evidence that a large sum of old coins was just moved:
https://blockchain.info/charts/bitcoin-days-destroyed-min-year
(I think people tend to over react to large bitcoin-days-destroyed events, however.)