@ Spikey 13
that's not just leaving, that's stalling the match. That's something completely different.
Reasons people leave (IMO):
1. They know they lost, and concede to their opponent as to not waste their opponent's time.
2. They know they lost, and concede to their opponent as to not waste their own time (they can only play for x mins/hours).
3. They know they lost because of a careless mistake and are embarrassed about their mistake (maybe?).
4. They know they lost and leave just because of their own reasons (non rage-quit).
5. They know they lost and are leaving just so their opponent can't get a K.O./other missions (how would your opponent even know if you're doing missions or not?).
I would say a majority fall into 1-2, a few being 3-4 while 5... although sometimes people are likely to do this, it's just stupid.
If you were really to rage against your opponent there are better ways to do it.
1. Stalling (as you have described by your opponent waiting out the clock)
2. Going to your opponent's presets/events and downrating (probably with multi-accs)
3. Trash talk them after the battle
4. Selling cards with 1 clint less than their cards (although for me personally, if it's a 1-5 clint difference I buy from the person who posted theirs first and don't have increased selling duration [Yeah, I'm weird. I know]).
5. Plenty of other ways, you can be creative.