Hello georic, there must be several reasons, such as: tiredness, recognition that the game has already been lost, rush to leave, phone call to answer, someone ringing the doorbell at home, bad mood, in short, a thousand and one reasons.
In any case, it is up to each player to leave the game in the way he wants, as long as he doesn't offend anyone or has the intention of giving up points, or something like that and as such we have to respect that decision of our opponent.
Good games and have fun
Compliments
Dav7d