Well, maybe it's because fanfiction can take the story in all kinds of wild directions. The gods in the Percy Jackson world are used to a certain order and set of rules. Fanfiction might rewrite their fates or show them in unflattering or unexpected ways that they can't control, so they're scared.
A plot could involve Goku being transported to a future where all his friends are dead and the world is in ruins. The unknown circumstances that led to this and the feeling of helplessness could make him scared. He then has to figure out how to go back in time and prevent this future from happening, all while being afraid of what he might find out about how this came to be.
One implication could be that the fanfiction has a real power of sorts. It can make these powerful, god - like entities feel threatened. It might also mean that the fanfiction is so creative and out - of - the - box that it challenges the very essence of the gods as they are presented in the original stories.
Yes, there are likely to be such fanfictions. In the Naruto series, Sakura can be quite assertive and strong - willed. Her temper and skills could potentially be a source of intimidation for Naruto in the creative interpretations of fanfiction writers.
Yes, sometimes Hulk is scared of Thanos. Thanos is a very powerful and intimidating villain, and even the mighty Hulk can feel intimidated by him in certain situations.
I can't say I'm scared. They might have suspenseful moments, but that doesn't mean they scare me. It depends on the way the stories are presented and my mood at the time.