Usually it is one of two things. Either the person is just a toxic asshole who wants to fuck over everyone, which is not that rare or they think of themselves as a player as well a bit too much. While the Dungeon Master is a part of the game and a player, sometimes the line can get a bit blurred where it seems like the Dungeon Master is playing against you to win. Does not mean that they're a bad dungeon master. Sometimes mistakes just happen or people get swept up, or other things they're going on. Soft reminders like saying that you enjoy all playing the game together or other such language that makes it seem cooperative helps to extinguish this behavior from a dungeon master. Using language like you're beating the dungeon master, even if it's in a joking way, can instill that behavior in the dungeon master themselves.