The emigration happens infrequently and I have always thought it was a result of client poaching. Put more politely, a client likes a lawyer’s background and abilities and persuades the lawyer to move over and take on a business role. The lawyer presumably welcomes the change of seat. The general counsel grins and bears the loss.
Then I read the opposite perspective in “From in-house lawyer to business counsel,” by the UK law firm Nabarro at 3. Said a general counsel, “It is quite difficult to persuade the business that it is a good thing for a lawyer to move into the commercial side.”
I never imagined a general counsel who wanted to offload a lawyer to the client side. Well, maybe in the rare case of a short grooming period for a likely successor, but not as a matter of course, not at all. To persuade a client to take on a lawyer sounds completely improbable.