What prompts Jem to tell his secrets about Boo Radley, in chapter 8? What is his motivation?

Jem was all shook up about the fire and the mysterious blanket wrapped around his sister. Scout described Jem's confession as a loss of his mind. He was actually trying to protect Boo.

Jem seemed to have lost his mind. He began pouring out our secrets right and left in total disregard for my safety if not for his own, omitting nothing, knot-hole, pants and all.

“…Mr. Nathan put cement in that tree, Atticus, an‘ he did it to stop us findin’ things—he’s crazy, I reckon, like they say, but Atticus, I swear to God he ain’t ever harmed us, he ain’t ever hurt us, he coulda cut my throat from ear to ear that night but he tried to mend my pants instead… he ain’t ever hurt us, Atticus—”