Shaking a brush as hard as your un-assisted wrist can shake it is not going to damage a brush that is properly glued in. There just isn't enough mass in the hair to generate enough force to pull a knot out of a handle unless there was a problem with the glue to begin with. It *will* however get more of the water out, and more importantly, the SOAPY water out, reducing/eliminating soap scum build-up in the knot.
Originally Posted by Matt555
It was just an un-fortunate "oops" in manufacturing, not getting quite enough glue in the knot. Unless the OP was "pulling" or squeezing, like as in "milking" the water from the knot, which would generate far more force than whipping it, not to mention not as effectively remove water from deep in the brush. He did use the word "PULL"... The only other thing I could think of that would be likely to cause separation would be using really hot water that either softened the glue, or caused the knot/glue/handle to expand/contract at a far different rate, separating the glue, but those kinds of temps will have detrimental long-term effects on any brush.
Brian - I think it would be totally inappropriate for me to even contemplate what I am thinking about.