Here is how it goes: You are playing a game, your opponent is playing of normal enough strength playing reasonable moves. Things are even or close to for a while... At some point in the game you do one of the following things:
1) Gain some sort of material
2) Gain a long-term permanent strategic advantage
3) Start breaking through the defenses
4) Any sort of move after which a computer would say you are either winning or solid advantage
There is still a bit of material on. It's not over yet. With solid defense, your opponent could maybe last another 30-40 moves before mate, but it's eventually probably gonna be there unless you make a horrible blunder or they somehow pull a miracle save.
It's at this point they shift into a mode similar to give away chess. Here take this piece, and that piece, and this other piece go ahead and mate me easy. They are playing a sort of help chess where they try to lose faster...
If your objective has become "Just lose as fast as you can" why continue to mate and not resign?
There is a custom that If your opponent has played a great game of chess you let them mate you and don't resign. This is one possible answer.
Another answer could be that you have mentally exhausted your opponent and they are now blundering and just refuse to give up because they just never give up.
Or maybe they just don't care about their elo
The psychology is not dissimilar to that of a 3 year old who says he will hold his breath until he turns blue, although with this difference: the 3 year old wants Mommy to feel guilty. The immature adult chess player is pretending that he doesn't care. He especially hopes you will think that maybe your initial gain of advantage was also due to his voluntaey suicidal play rather than your having beaten him by being a better chess player. There is no way you will think that, but he has the irrational hope because, well, he is irrational, and basically infantile. The truth hurts, but the infant consoles himself by sucking his thumb, as it were, and pretending the truth wasn't true.
*voluntary
Man, they're just on tilt. Don't try to understand that. Some insult toy, some post something like "My last defeat against a cheater", some fall in a small depression.
@nayf I think you may have hit the nail on the head. That is a very plausible explanation.
You know, when you were a kid, you'd meet these infantile types all the time in sports or other competitive endeavours. After beating them: "I wasn't trying". Recognising the phenomenon is no big deal. The surprise is that some are still around in adulthood. They never learnt to cope with losing. The diagnosis is easy; identifying the cause or causes is more challenging. Who knows, maybe they sucked their mommy's titty for too long, or were spoiled or babied in some other way.
I have done this. It was the equivalent of banging my hand on the chess board so hard that the pieces would all fly hither and thither. At least I would get the enjoyment of blowing things up.
For my part it was anger and frustration at my poor play. I play better now and don't do this, but when i did I never did it as an insult to my opponent.
Whither you sent the pieces, be it hither or thither and even without dither, it was an immature practice stemming from an umbilical cord needing to be severed with a scither.
Nayf that all sounds like unfounded speculation honestly. In the situations described in the OP, I feel like sometimes I get into the situation described, and feel like the game is over, so I try to simplify the position massively by trading things off, that way I can get onto the next game faster, and see if I have a chance to go for a draw.