Brain Drain

What is the cause of a brain drain?   Could it be something like this?: Two-thirds of millionaires left Britain to avoid 50p tax rate

The figures have been seized upon by the Conservatives to claim that increasing the highest rate of tax actually led to a loss in revenues for the Government.

Also, notice the phrasing in the above quote.  It is as if it is a doubt that such outcomes will occur.  Yet it has occurred.  It is almost if the occurrence was some sort of mystery that cannot be accounted for.

It should be considered as common sense, but common sense is uncommon now.