Could it be someone at bitfountain with access to those coins is moving them to a safer address, just in case?
If someone stole the privatekey from fc they would take all and not leave 10k BTC behind. This is all fucking weird

Each original owner of bitfointain has their own dividend address, this being the largest, most likely the personal address of FC. The fact that it wasn't immediately drained all at once implies that whomever is moving the funds isn't worried about anyone else having access to the private keys.
I can only think of two scenarios that would make sense and neither are good.