What secrets is Jem moved to reveal to Atticus? Why?

Jem is so unraveled by not knowing where the blanket around Scout's shoulders came from that he tells their father everything they've ever done to get Boo's attention..... he tells about the gifts in the knot-hole and the mending of his pants. All jem wanted was to keep trouble from finding 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—”

Atticus said, “Whoa, son,” so gently that I was greatly heartened. It was obvious that he had not followed a word Jem said, for all Atticus said was, “You’re right. We’d better keep this and the blanket to ourselves. Someday, maybe, Scout can thank him for covering her up.”