running down the clock?

So I've only really been playing chess since about september, and as I've gotten higher rated I've noticed way less people resign really early (good) you can usually come back from a blunder.

But I've also noticed a tendency of players to seemingly get up and walk away when there's an imminent checkmate?

eg this game I played recently - I can't really understand the purpose?

Are people hoping that I'll get bored and resign so they can preserve their rating? or that I'll stop paying attention and they can win on time?

Honestly it just feels kind of rude? Idk I guess this is just the 1000 rated version of resigning the second you blunder a pawn but it seems like poor sportsmanship or whatever. 

Have any of you noticed this? do you have any insight? 

It's a boring (stupid) behaviour. Don't care, play fair and enjoy.

well i understand the frustration factor and making you wait for your points -- when they are clearly lost -- they too have a self frustration factor going on also and are looking at the board wondering why they lost and maybe talking to you about it which happens by me alot ---- and really it is bad manners but then it is their time and to do what they want --- leaving a game unattended is a different manner -- that is plain rude -- in what ever min game your in they should stay and take their lumps to the end it said above that it happens less in higher rated games i believe that or in longer games where the clock kicks in and ends the game after so many seconds -- it is what it is -- i do it sometime myself and i feel bad afterwards especially when the person won't engage in conversation about their win they have over me-- they just sit there like a lump waiting for the the clock to run down or the end game feature kick in -- both sides are really wrong and rude but then war is hell don't they say ?