Why does Jem tremble when he comes back from getting his pants in Harper Lee's To Kill a Mockingbird?

By the end of Chapter 6, Jem trembles because he has experienced a shock retrieving his pants from the barbed wire fence surrounding the Radleys' house. In the beginning of Chapter 7, Jem explains to Scout that his shock stemmed from feeling constantly watched.

In Chapter 7, Jem explains to Scout that, when he returned to the Radleys' property to retrieve his pants, he did not find them as he had left them. When his pants caught on the barbed wire fence, Jem had to get them off as quickly as possible and leave them in a tangled heap under the fence. When Jem went back that night to retrieve them, however, he found them "folded across the fence. . . like they were expectin' me." His feeling that someone had expected him to retrieve his pants makes him tremble that night.

Jem continues to explain that, not only had someone folded them neatly and laid them on top of the fence, someone had also sewn up the tear but in a crooked, disorderly fashion. Because Jem found his poorly-mended pants waiting for him, Jem felt like someone, namely Arthur (Boo) Radley, has been watching him:

Like somebody was readin' my mind. . . like somebody could tell what I was gonna do. Can't anybody tell what I'm gonna do lest they know me, can they, Scout?

If Arthur has grown to know Jem to the extent that he can predict that Jem will retrieve his pants to hide the evidence from Atticus of his misdeed, then Arthur could only have gained this knowledge by observing Jem and closely watching and listening to his conversations. This realization makes Jem feel spooked to the point that he trembles the night he finds his pants.

As the chapter progresses, however, Jem and Scout begin finding more and more things in the knothole, and Jem comes to realize they are gifts from Arthur. The realization that Arthur is leaving them gifts helps Jem see that Arthur is not observing Jem out of some malicious intent but because, in his own quiet way, Arthur wants to know the children and be involved in their lives.