Yeah this has been a known problem since the XB1 launched, although it's becoming more prevalent lately. My friend fell victim to it because he is extremely good at Halo and was stomping people in Halo:TMCC. In a matter of days his rep went from 'good' to 'avoid'.
More recently, just over the past few days my rep has dropped from 'good' to 'needs improvement'. I know why, because I've only been playing one game. Apparently people don't like Trickster Rogues, especially the ones who kill them a lot, in Neverwinter PvP. I've been consistently going anywhere from 10 and 1 to 30 and 0, averaging around a 15:1 k/d playing on my rogue. I think I had a string of 8 or 10 matches without a single death. And that also just happened to be the day (yesterday) that I noticed my rep had dropped to 'needs improvement'.
I'm not going to lie and say I've never sent spiteful message to other players, because I have a few times, but those instances were reserved specifically for players using what are, in my opinion anyway, broken builds that the devs really need to nerf/patch out of the game. Rogues are already a massively overpowered class even with a basic PvE/glass cannon build. But there are permanent-stealth and chain-stun builds that make rogues not just overpowered, but downright broken. I have zero respect for people using those builds, and I've gotten into heated discussions with a few of them about it considering my experience in how overpowered I already feel playing as a 'normal' rogue. A couple of said conversations actually went quite well and the players I talked with agreed that it's a broken build but were content using it until if/when it's patched/fixed. But the others... lets just say I'm not proud of some of my messages. But out of the thousands of players I've played against, there have only been a handful that I've said not-so-nice things to, so I know for certain that isn't what led to the negative rep. It's largely just from killing people too much, too easily.
Wanna hear the really sad thing? I actually made attempts on several occasions to be sporting about it. If my team was trouncing the opponents, I would go into /zone and /say channels and state that I was backing off to allow the opponents to capture control points to at least gain SOME glory from the match (glory being the game's PvP currency). I would ask my teammates to do the same. A few times my team cooperated, and the opponents would (sometimes) thank us for the 'mercy rule' mentality, so they could get something out of the match. More often than not, though, my teammates would continue spawn-trapping the opponents once we got the upper hand, and I'd be left to my devices jumping around in circles on my mount because I felt like it was overkill to continue spawn-trapping/spawn-killing people needlessly.
My point is that despite my best intentions to behave sportingly, I still got enough negative feedback, presumably by people blocking me, that my rep fell, just because I choose to play a class that is generally disliked and become quite good at said class. I've seen plenty of rogues that barely break even or even go negative k/d. I highly doubt they're receiving much negative feedback. Likewise, I've seen hunters and clerics that are a massive nuisance, one being a hunter I team up with sometimes, and it just so happens his feedback has gone down too from playing 'too well' in Neverwinter. So it apparently isn't just restricted to rogues, but rather to skilled players.
People abuse the rep system unfortunately. They don't even have to report you though, and might not even realize they're abusing it. If all they do is 'block' you in the hopes that they won't run into you again in matchmaking (kinda the whole point of the system originally, to get people more at your skill level), it still gives you negative rep. The 'report' function is something that's supposed to go higher up the XBL chain and potentially lead to people getting banned if they are regularly sending harassing message to people or other such nonsense, but from what I've noticed the 'report' function rarely does anything, although it may lead to bad rep as well. But as I said, if you excel at any particular PvP/versus mode, and enough salty crybabies 'block' you out of fear of facing you again, your rep can and will drop. The solution is to play something else with versus matchmaking where you don't care about winning, or are legitimately bad, enough that your rep goes back up if you're very concerned about it.