頁 originally depicted a person with an emphasized head, and it came to mean 'head' in classical Chinese. Its use for 'page' or 'leaf' in Japanese is a later semantic extension, possibly from the idea of a flat surface like a head or a leaf.
Meaning mnemonic
The top part 貝 looks like a flat shell, and the lower strokes ハ and 目 suggest a leaf or a page spread open. Imagine a book opened flat like a shell, each side a page.
Reading mnemonic
For ケツ, picture a page from a notebook getting a small ketchup stain: ketchup ->ケツ, and the page is marked.
