When they have historically sold coins, they sold blocks of coins to bidders so it was all handled off-exchange and didn't affect pricing. I hope they are just using Coinbase for some custodial reasons, rather than having the intentions of actually dumping all of the coins into the open market as this could impact pricing. My suspicions on why they have deposited the coins into an exchange, is because they don't want a repeat of last time where two US federal agents tried to steal the BTC for themselves, so the exchange would have more controls established and completely disallow withdrawals etc. Coinbase is also a US company so there is probably some sort of safety provided to the US government for the storage. I imagine Coinbase representatives may be advising the US government on what to do also.