While the attached article is specifically Creative-focused, the point raised about the increasing tendency to see top talent move in-house rings true for media agencies as well. From experience, roughly 30-40% of actively job-seeking candidates in this space prioritize client-side opportunities over a more senior role in another agency, meaning the biggest challenge those companies face in attracting and retaining great people isn't from other media agencies but from their own clients and their competitors.  

The how and why of this trend are quite complex, and can be different depending on the individual, but often come down to a desire to be 'closer to the business challenge' of a particular brand. Ironically, this can regularly be a reason why someone working client-side wants to move back into an agency role - they often find that the work being done in-house is too narrow, too bound to sales figures to allow for truly strategic, cutting-edge planning.

Whatever the reasons for the mass desire to move in-house, agencies clearly must find a way to engage their best talent and make them feel like the very best work is being done on their side of the fence.