I worked for a software dev company before, quite well known, and in my contract was a specific clause stating *anything* I produced, no matter where and when, would be considered the intellectual property of the company.
They were pretty cool guys in general so I questioned the motivation behind it some months into my employment. Turns out they'd been burned badly before by having some of their assets reused and sold off, and henceforth used that clause as a contractual safety net. In practise however, things were generally totally open and innovative with a lot of devs working on separate projects; the enforcement of this clause depended on your own standing and relationship with the company.
In other words, it's very possible he had this in his contract and for precisely the same reasons this case has gone to court.