From what I understand, it was mainly the way Michael ran the project. Some developers tried to (forcefully) take over the project, but in the end they had to fork it. Here is a recent article with some comparisons, now that it's been four years https://lwn.net/Articles/650816/.

There are two questions now that Michael has left, will they still actively merge libav patches (which he was mainly doing), and will development continue at the same pace.