A project being GPL'd doesn't necessarily mean the copyright isn't owned by one party.
He's clearly the top contributor, but not the sole contributor.
Sadly, it's why there's more and more projects adopting CLAs. Some that say that all the code you're contributing is owned by the project and not yourself. Allows for future relicensing in some cases.
You're conflating CLA and CTA/CAA, the difference is explained here [1], and it matters.
[1] https://opensource.stackexchange.com/questions/860/what-is-t...
In that case it looks like most of the other contributors contributed after the CLA was added but there are still a few commits from before that point. Accordingly he should have sought permission from those people before changing the license.
I contributed a few minor fixes and got an email from David asking for permission to re-license.