The higher the tax rate is on talent, the higher the incentives for talent to move where it's most financially advantageous. I know it sounds obvious, but it's the truth. It doesn't mean that everyone's going to move away. That's not at all what happens, but you just need a few to make a dent in your expected tax revenues from the hike, and that's enough to lower the tax base. It's enough to sometimes create a shortfall or sometimes create tax revenues that are negligible, compared to what they would have been without a behavioural response.
On February 19th, 2016. See this statement in context.