Ok, found the answer: https://twitter.com/BitwiseInvest/status/1757813223536103568

tl;dr: they're upgrading to native segwit.

patrick1

About an hour ago all of the bitcoin (over 18k bitcoin) were moved out of the wallet. Does anyone know what is going on?

It turns out they're just updating to native segwit, 

https://twitter.com/BitwiseInvest/status/1757813223536103568

bitcoin

~About an hour ago all of the bitcoin (over 18k bitcoin) were moved out of the wallet. Does anyone know what is going on?~

It turns out they're just updating to native segwit, https://twitter.com/BitwiseInvest/status/1757813223536103568