Well, it is not as much it is people from Kazzak, as a lot of people on Kazzak seemingly have certain traits. You will often see such behaviour trends in your m+ runs as well. Not all, mind you, perhaps as low as 4 in 10. Kazzak players especially when they come as a pair is taking a risk you should not take in m+, as their bar for leaving is lower than most.
While I will recieve pushback, and justifiably so, as the mantra ‘some but not all!’ and ‘this rings true to all!’ rings with at least some justification of argument. It is also true that if you invite players from certain servers, there are certain expectations to what to come based on previous experiences.
So even if it is truly only, let us say, one in ten, from the server ToxicTestRealm that does leave keys early and for no reason - that is the experience your brain will remember. Not the nine other times that was without issue. So keep this in mind.
This also goes the opposite direction of things. If I see a Magtheridon player, I am basically shook and think “Oh it is a unicorn.” My expectations from experience is that it will be a solid player, with chill attitude and one that just gets things done. The rarity of the players from there is what makes it oh so special. Silvermoon, I expect things to get done and well, yet I also expect a certain level of, shall we say - elitism? Twisting Nether got those old school players, that seem to somehow still abide to the old rules of dungoneering together (You stay till we agree to leave). Tarren Mill is Silvermoon, yet a lot more humble, but they do have a mouth when they feel wronged - but all meant well.
I could go on, but you get my drift. All servers leave a impression based on the player interactions you had with the realm. It is all very subjective and is nothing but a fun little thing to take note of as the feel builds.
Should you avoid players from certain servers? No, there is no need. Exceptions are not the rule. Yet, would I avoid certain servers based on previous experiences to increase the chance to not get leavers? Myes. I think I will take those lads from Ravencrest, why? I am superstitious and as they have not left one of my keys before, I just presume this next set wont as well.
Also, I am the type that roll a dice before inviting a protection paladin, because that dice roll in my head saves me from the worst of the ragequiting archetype players of that class. Stupid, a ritual only - yet it help keep me sane, and the bad paladins away.
Take my words with a bucketload of salt, gathered from my failed keys and raids. Now let us see some other takes to smile about. Because in the end, it is not a serious matter, this is a matter of feel and is silly - have fun with it 