Based on majority of MMO/Multiplayer there are few common factors that encourage players to leave the game forever(as opposed to temporary).

A."Grind Ceiling" :Time investment required for reaching reward grows exponentially instead of linearly. Usually magnified due fact players can pay real currency to avoid time investment(cash shop shortcut).
Typical discovery by long-time players when they reach endgame content.

B."The Big Nerf":Game has introduced/redesigned a mechanic that devalues many builds/characters and provides no alternative. Players that invested resources in their characters give up.
Subtle variants: Key skill mechanics changed to less effective form, angering players dependent on the skill/spell/mechanic in question - their option is either switching class or leaving the game(most players have a favorite class supporting their gameplay style).

C."Cash Grab": Game has reduced previously available items/rewards to encourage cash shop items. Usually considered a blatant break with status quo of the game.
Subtle variants:Grind/farming nerfed to increase cash shop value. Typically after the game has amassed a large playerbase, the management decides to reduce economic input(loot) hoping "the free auction/trade" will fix it(redistribute the more rare loot).

D."Immersion-breaking bug" :A game-breaking bug: Some update has introduced a bug that created new mechanics/features/exploits that are not fixed in time, creating a new meta-game that most players are unwilling to participate in(immersion break).
Subtle variants: Design flaws that break immersion in multiple places, such as forcing attention from player to non-world content(UI) or "breaking the fourth wall" in

E.Forced content/mechanic/rule pushed to players without it being (organic) part of the game.
General the forced content feels out of place or artificially inserted into the game.
1.Forced party and related forced socialization.
2.Forced repeatable quests/puzzles to access content.
3.Repeated content in general, that has no replay value.
4.Unwritten rules or non-game laws that players are forced to use.(e.g. you can't use X with Y due etiquette or the moderator/gamemaster will ban you).
This falls into "intentional immersion-breaking" that limits freedom of player.