Yeah, it was probably a flagrant. One thing I hate about the NBA is that the refs base calls like this on the size of the two players. If BBD fouls somebody, making a play on the ball, he may get accessed a flagrant for "excessive contact" just because he's a big dude, and caused a big collision. A small guy like Rondo, fouling a guy like Miller, is less likely to be called.
If Miller had made the *exact* same play on Rondo, with the exact same amount of contact, Rondo would have went sprawling, Miller would have been ejected, and Celtics fans would be calling for his head.