In "To Kill a Mockingbird" even though Dill gets permission to stay, why does Scout say their peace has ended after a week?

In Chapter 15, it is after a week of Dill being in town with that the events surrounding the trial of Tom Robinson really pick up.  So, the lack of peace has nothing to do with Dill himself; he just happened to show up before things really got intense.  It is a week after Dill arrives that Heck Tate and a group of men come to the Finch house and try to convince Atticus that Tom shouldn't stay in the Maycomb jail that weekend because they are worried that there might be a riot or some sort of violence.  Atticus refuses, and then goes and plants himself outside of the jailhouse, just to keep an eye on things.  An angry mob does in fact show up, and it could have gotten really scary if Scout hadn't unintentionally broken the tension by talking to Mr. Cunningham.

So, Dill's arrival just happens to be right before the trial, which Scout said was "a nightmare".