I don't think there was any discussion by Canonical of the choice. It occurred shortly after the Harmony agreements were released. The interesting thing about the choice is that Canonical chose not to pick a copyright assignment Harmony variety, which would have been the closest thing to its earlier contributor agreement. I suspect this is because Canonical realized that there was some nontrivial subset of critics of its copyright assignment agreement who might find a comparable CLA more palatable, and also that such a CLA gave Canonical just about everything it thought it was getting out of the copyright assignment agreement. But this admittedly is speculation.