It indeed would be interesting to hear their reasons. One thing I know that business world is using a lot of open source software, and GPL and to some extent LGPL has some negative reputation there due to legal issues its usage may bring. Especially with something like PHP library, where concepts of derivative works for bigger application that is distributed together with Doctrine may be less than clear. So I can see how that can be a limiting factor for them and a plausible reason for the change.