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)
Yes and I think the answer to your 2nd guess would be that the owner believes that it would continue to grow since back then there are so many speculation and we all thought that it was going to continue.
And the third one would be a good one too we don't know if the real owner still knows about their crypto wallet and might have forgotten about it.