Employee's start to lose respect for a company when senior bosses leave and are replaced with someone who is not of the same world as those they are managing. Jony Ive is a designer, his education and employment history has all been in the design world. So when he was made the boss of Apple's designed department, all of those in the design team would have respected his appointment because he was of the design world, he had a designers mind and therefore could understand the minds of other designers.
One of the hardest thing to accept when your a designer is having your idea/design rejected or dismissed. Whilst it would still be hard to accept, having someone of I've's experience making that decision is easier to accept because it is a very experience designer rejecting the work/idea of a least experience designer. That dynamic changes when you put a non designer in charge of a design team. When idea's and designs get rejected by the non designer, the designer starts to question the ability of the non designer boss to make the right decisions because they are not of the designer world. This happens time and time again in the employment world, employees getting fed up because their new boss is somebody not of their world and therefore does not understand them or the job at hand. This starts to cause frustration and anger with the employee's, they also start to feel underappreciated because their boss does not understand the world they work in.
I am sure if those leaving the design team since Ive left were honest enough they would say they left mainly because their new boss was not of their world and therefore would not be able to relate to the problems and issues that designers face.
I've seen it personally happen during my time at working at different manufacturing companies, an engineering boss leaves to be replaced with a manufacturing boss, engineers start to leave. Quality control boss leaves and is replaced by a sales boss, the QC people start to leave and so on and so on.
Until Apple get's another highly experienced designer in as the new design team boss, designers will keep on leaving the design team.