From Steve Hardin's bio on Wikipedia:
From: https://en.wikipedia.org/wiki/Steve_Hardin" onclick="window.open(this.href);return false;While back in L.A. with Steppenwolf (late seventies?), Hardin learned that Glen Campbell was wanting to hire a full-time writer for his records, tours, and T.V. performances. He interviewed, got the job, and spent the next five years working with Glen writing songs (using the name Joe Rainey), touring, and performing on The Glen Campbell Music Show.
As Mike mentioned, a pseudonym could have been used for contractual reasons or as part of a financial arrangement (for royalty reasons)?
From BMI's website:
From: http://www.bmi.com/creators/detail/song ... _copyright" onclick="window.open(this.href);return false;Q: If someone hires me to write a song for them, who owns the copyright and how long does it run?
A: For such “works made for hire,” as they are called, the employer is considered the author, and the copyright, which the employer owns, runs for either 95 years from the time the work was first published or 120 years from the time you created it, whichever is shorter. (That is the same copyright term for an anonymous or pseudonymous work.) In order for the work to be considered “made for hire,” it must be prepared by an employee in the scope of his employment under a traditional employee-employer relationship, or else it must be commissioned as one of several special categories of works (such as part of a motion picture or other audio/visual work) with there being a written agreement between the parties that it is to be for hire.