One question OP, how did you 'specifically' found this addresses?
I am guessing you have been monitoring it before now to know when the owner made this move.
Anyways, three particular thing is involved here;
1) Perhaps the owner suddenly remembered he had some certain amount of Bitcoin
2) The owner has been hodling since and probably finally decided to make use of the funds (Not sure why he didn't spend not even one when the price was 20k though)
3) Someone else got hold of the private key or assess to the funds. (I hope this is not the case here)
I have a list with dormant addresses around 2700 and I'm monitoring their activity for personal reasons.
Few days back I checked the balance on all of them and I saw that the funds were moved for some.
We don't know if is one person or a group. Also we don't know if is the owner..but it has the private keys.
The funds were only moved to other addresses.
Maybe this has been done by a single person, as you said this wallet already been existed since 2011. I think an individual store it and do not intend to move or use it over the past few years for his own reasons. This may be his time to make a move.
Well, we don't know his reasons.