Maybe they are disappointed with the animation quality or the way the story progresses. Sometimes, it's because it doesn't live up to their previous experiences with similar anime comics.
It could be because the plot doesn't go the way they expected or the characters don't act as they wanted.
Maybe they just don't get the style or the story. Different strokes for different folks.
Because they find the humor and antics in the comics really funny. Simple as that!
It could be because they have a deep attachment to the original manga and any changes feel like a betrayal of their expectations.
Maybe they don't like the plot or the way characters are developed.
People might get nervous watching cartoons if the themes are too intense or if they have had bad experiences related to cartoons in the past. Plus, if they're really invested in the characters and something bad happens to them, that can cause nerves too.
Well, it could be because of creepy characters or intense storylines. Sometimes the animations and sounds can be really spooky.
It could be because of staring at the screen for too long without breaks. That strains the eyes and causes soreness.
Maybe it's because they've been watching for too long and their eyes and mind get exhausted.
Maybe the storylines are too predictable or the characters aren't developed well. That can make readers angry.
It could be because they're sitting in one position for too long and it strains their stomach. Or maybe they're eating junk food while watching.