Because he didn't like PZ's book review. Because PZ called him a "classic crackpot".
That's it. He didn't like the assessment, so he goes ahead and proves that it is appropriate; he acts like a "classic crackpot."
Blake Stacey has the story, updated as new comments come in from other blogs.
PZ, in one of his two book reviews (first edition, second edition of the book), shows a sample of his "research": an illustration of the development of a spider.
Here it is; you be the judge.
I had never seen before a spider with 10 legs. Here's a real spider, with all those legs clearly visible:
Pivar's spider is not as accurate as this one (from an IIDB post):
At least they got the number of legs right. And, by the way, the number of legs is the first piece of information in any definition of the word, "spider".
From Encarta, for example:
spi·der plural spi·dersStuart Pivar is a crackpot.
Definition: 1. eight-legged animal that spins webs: