Comment by FiddlerClamp

I think it was a one-two punch:

1. Snoopy becoming Flanderized, as in the "Happiness is a warm puppy" stuff from the 1960s.

2. Introduction of Woodstock the bird. That meant Snoopy and Woodstock went off and had their own adventures which didn't involve the human gang at all.

I also wonder whether Schulz participated in any recreational drugs in the 1960s. I don't meant to be disrespectful at all, but some of the stuff he drew was pretty wild.

There's a set of strips where Charlie Brown sees the moon as a baseball (and later, Alfred E. Neuman's head), another where Snoopy dreams of Charlie Brown flying him like a kite and him crashing to the ground in pieces, and a horror-movie-like series where Linus's blanket attacks Lucy. All very strange.