Wouldn't it be cool to give everyone their own story?
Personally, my head cannon says Grounder is only attacking the player because Scratch isn't there to help relieve him.
Unless you meant for the player characters, I find it fun to fill out the reasons being for those little characters first, then fit in the big characters last since they make the largest impact.
Like, for sure Spiders would be in the forest. Why wouldn't they? It's their home. But we can make that better.
We can say, a parade of arachnid warriors accrued combat wounds (They all look like they've been stabbed) attacking the denizens of the forest, when the Pink Cloud attacked and turned them all into what they are.
Once that little tidbit is in place, we can say Rouge as been attempting to parlay a peace between the Arachnid Vagabonds when you play as her in Scene 2, (Act 2? Let's just say forest.) since she works for G.U.N. and all. Not entirely sure what they do, but something something it's part of her job.
Even Tails could have a say in this, since he was a natural at keeping peace, he would have wanted to teach these Spider Warriors a lesson anyway -- though, I don't think I've seen him actively attack something other than a robot, it would be out of character for him to do so on a living creature. He'd be more of a mediator. But since they are now clouded by the pink smog, Tails might want to try and find the General/Queen and see if there is
something he can do before the army is lost of it's control.
Cream -- She meets' the Spider leader and it turns into a fetch quest! Sure, need to have one of those right?
Could think of other things for the other major characters, but this is just examples by filling in what a minor character is there for, and implementing this idea across multiple major characters as they are all influenced by this one gear in the system.
