I think this is case of top posting, because messages are sorted newest first. But this one is paginated.
I will refer to it as "newest first".
Well, it has problems. If a new message arrives, content on all pages will be shifted by one page, so :
1. It wastes bandwidth
You are on page 13, You sip a coffee and click to the next page. You : "What? I have seen these messages already. Probably double post ". That is a case when someone added messages while You were sipping cofee.
2. You might miss messages
You have not been there for a while. So You click page 24, guessing it will be the time. You sip cofee, and decide to go to 23; while You are sipping cofee somebody added messages.
3. You can not hyperlink to page
"What a funny cat" You say and want to share the picture with gf; You link page 26. Later that day she clicks on a link and she has another cat on page 26.
Yes, You can link to image in this case. Some other times You have to link to whole page.
4. It wastes time
Reloading takes time. Finding out You have seen it and need to reload takes time. Realizing page can not be linked takes time; looking for image url takes time and pastebinning text takes time (or saving a page and sending it takes time).
5. Comparison with "oldest first"
Reason for "newer first" might be, that user gets the newest stuff by clicking at page 1. In "oldest first" You can get newest stuff by clicking on the last (biggest) number.
In "oldest first" pages do not shift. With exception of messages deleted, but it can be overriden (database can contain "message X is on page Y" relation so pages will not change). Therefore You can link to page in "oldest first".
TL;DR "newest first" has some problems, "oldest first" does not have these problems.
To me, it is all sometimes annoying. Some other people I ask don't mind it much.
I don't see reason to use it; I do not see benefit. I have no clue why some pages use it. Perhaps I miss how is this approach good.
Why is it used more?
Why is it preferred at certain cases?
Is it somehow better? Is this more efficient? Is it easier to code?