This post seems to provide evidence that people who do not traditionally write textbooks (not the top hundred faculty members in a field) are being afforded a new opportunity by textbook companies to "write a book", which either means "pick and choose some portions of the standard textbook" (discussed elsewhere in the comments) or "publish a 'course reader'-style collection of readings as a book" (discussed in this blog post).
It seems to me that this new opportunity also represents a new revenue stream for more faculty members than currently receive textbook kickbacks.
I don't mean to suggest that I have any special knowledge of the motives of adjunct professors in particular — basically I just read the Chronicle of Higher Ed like everyone else. This is a well-trod area: see July 2008's "'Custom' Textbooks Raise Money -- and Questions of Ethics" (http://chronicle.com/article/Custom-Textbooks-Raise/41288), February 2010's "Format War Heats Up Among Publishers of Electronic Textbooks" (http://chronicle.com/article/Format-War-Heats-Up-Among/64323...), or October 2010's "As Textbooks Go Digital, Will Professors Build Their Own Books?" (http://chronicle.com/article/As-Textbooks-Go-Digital-Will/12...), to pick a few examples.