It could be a metaphor for how problems tend to accumulate when one bad thing happens. Like if you lose your job and then your car breaks down all at once.
Maybe it's because the rain creates certain conditions that are favorable for the monster. Like, it could be that the moisture or the sound of the rain triggers something in the monster.
Well, that could involve a variety of plot developments. Maybe a big reveal, a new character introduction, or a crucial decision by one of the main characters. But without reading it, it's hard to say for certain.
Overall, the story in 'When It Rains Worm' fanfic is full of mystery and adventure. The author creates a tense atmosphere where the rain of worms brings various challenges and opportunities for the characters. There are also elements of friendship and courage that drive the narrative forward.