Let’s take a look at the change after that sustained high frequency section.

That small local low spot at 34,600 indicates the end of Chapter 6, Out of the Frying-Pan Into the Fire, when the excitement of goblins, wargs, and eagle-rides is ending.  The local peak immediately after it at 37,000 is the moment we meet Beorn.

The story continues to fall off in use of uncommon words until the nadir, which deserves its own post.