It’s aggravating to a lot of players. Though APM can only be considered as a hint regarding that. A total career games too low for their leagues however, can be considered as a confirmation. According to my sources, the smallest amount of games to reach diamond was 200, and that was for a guy who was both coached and gifted.
In other words, a guy who reaches diamond MMR with only 50 game into his profile can’t be anything but a smurf. He has already learned the game elsewhere. 
It’s certainly not the devs. As for why 50 games (supposing it’s not a pure coincidence), 25 games are the amount of games required to get out of provisional MMR state : you get placed accurately only once you’re out of that phase. And some smurfs do make sure to never go over that amount, to appear being in a lower league than they are. More about those smurfs in the dedicated thread (part I & II, or search for « Non freelosing smurfs » in your browser).
So it’s possible that you met some smurfs who pulled that little stunt twice. Either that, or a coincidence. AFAIK, Alphastar (Deepmind’s SC2 AI) isn’t testing on the ladder anymore.
Some players just can’t stand the frustration or stress that competition induces anymore. Others just can’t stand a match-up. And there are some who revel in trolling supposedly vulnerable targets. Some do that upon restarting the game after a few months break, to soften the resuming. There are quantities of reasons behind that.
Not that those reasons are necessarily legit of course, but they’re still a cause.
I’ve done a sometimes hilarious, sometimes disillusioning survey about those reasons among smurfs. If some are interested, I might publish it once my leagues reports are finished, but it was primarily done for the devs, in case they wanted to address the causes. 