If you rank points debt yourself you rank farther down at the end of seasons I believe.
If this is true I suppose it would apply to manual rank resets as well
ok I think I know the answer. If someone grinds the game a lot and is S rank, they will eventually get a lucky streak and land in S+. If they are truly an S rank player then they will lose most of their games, and if they keep grinding the game without rank resetting then they will get a lot of debt (and more importantly, losing most of their games will decrease their internal glicko rating) so when the new season starts or they finally decide to rank reset, the game puts them at a very low rank. This feels more likely to me than smurfing because it's just someone trying to grind the game. Although it's possible there are people intentionally doing it as well.
While it could very well be people who got ranked down, it has been a thing since the first season where you could get paired with people from other ranks, such as people in B getting matched against S+. Haven't seen that happen in a while, mostly because not as many people are playing Series as they were during the first season (or could of been patched later and not aware of it).
I think this is because at the beginning of the game, people's ranks didn't correlate to their actual skill very well. The game took everyone who was B- to X in splatoon 2 and stuck them all in B-. So someone who had gotten back up to S+ in splatoon 3 might be a similar skill level to someone who was still B rank in splatoon 3. So the game would match them together because they were similarly skilled. This isn't an issue now because most players aren't entering the ranked system for the first time anymore.
The reason (according to my theory) that the S rank players I mentioned above might get put all the way down in C rank is that when they repeatedly lose games, splatoon decreases their glicko rating, so when it redetermines what rank they should be it puts them low.