Things start to appear in the trees at the Radley Place in To Kill a Mockingbird. What is going on at this time?

In Chapter 7 of To Kill a
Mockingbird, after Boo has repaired Jem's pants, the children find a ball of
grey twine, which they leave in the knothole of the tree for a few days or so.  But,
after Jem and Scout remove it, little soap figures of themselves appear.  Clearly, Boo
Radley extends a part of himself tentatively to the children.  For, in his lonely and
alienated existence, he wishes to share some human contact.  Unfortunately, after Jem
and Scout write a thank-you note, Nathan Radley, Boo's brother, cements the hole, giving
the excuse that the tree was sick.  Ironically, a knothole in a tree indicates 
sensitivity and a willingness to give.

This scene from the
first part establishes the credibility of Boo Radley's final actions.  Having previously
extended his love, then, in the last chapter, it does not seem so unusual for Boo to
rescue the children from harm at the hands of Bob Ewell.