At 7:20 UTC, backlog still large:
2023-01-02 07:04:54 24,468 1,156,179 525,948,678 2023-01-01 21:02:06
(0d 10h 02m 48s ago)
I have noticed that several folks do indeed upload at long intervals (3 months, six months, a year). Perhaps year end is a particularly fruitful time. I have only noticed particular DX with this habit. But, it is probably fairly widespread.
Everyone has their own strategy.
How many on-line log services does one join? The most popular are LOTW, Clublog, eQSL, QRZ.com's log and Ham Radio Deluxe (probably about in that order).
How frequently does one upload to these?
Clublog is usually uploaded fairly often because DX stations use frequent uploads as a "dupe" reduction strategy. That isn't everyone, but it keeps the numbers down.
Some upload to LOTW with an eye to QSL management. More frequent uploads cuts down on the number of "buro" requests (there are a substantial fraction of people who, when they learn DX1DX is an LOTW user will wait before sending any kind of card). Less frequent uploads results in more direct requests.
I have processed some of my lotwreport.adi and these suggest it is on a rough continuum -- you'll get uploads for a QSO years later sometime. But, the majority that ever happen are within 90 to 120 days after the QSO and quite a decent fraction during the first week after the QSO. Still, there is a long tail.
This would also apply to other services.
Not everyone cares about (e.g.) eQSL or QRZ.com awards programs. Such individuals may upload to those sources primarily as an occasional courtesy. Many contesters probably do the same with LOTW. Who knows how many resident DX (who may already have all the ARRL awards they want) do the same?
So, some of this is probably a kind of "triple witching hour" where the annual, semi-annual, and quarterly uploaders are all uploading at year end.
There are several ham web interfaces I have worked with recently. Many of these suggest that the big cost is connecting at all and until the volume of data gets really insane, a small amount and a big amount aren't much different in cost. Still, a huge volume of requests is a huge volume of requests.