Wow, Treehouseman, that's . . . an awfully negative perception of it. And kind of fascinating that that is where your mind went.
Every agent I've had who moved agencies did so because the new agency was better, paid better, gave them a better job title etc. And it seems in publishing in general there is a ton of moving around. I've had editors leave, publicists leave, and agents switch agencies. None of these people did any of the things you are suggesting they did. It's not suspicious for people to move around in publishing. It's very very normal. And if you want to look at it as nothing but a negative then you probably need to get out of publishing because there is no stability and it's very very rare anyone stays put. Publishing is a tough business, as authors but also on the other side of things. Yes it pays extremely well to the people at the top of the food chain (including a select few authors) but the rest of us are doing it more for the love than the big bucks. People are looking for the best possibly place for themselves and if they sense that there is no further upwards mobility, or that their job has gone stagnant, they look for the next opportunity. Like most people who work for a living.
(I should note that an agent switching agencies doesn't necessarily mean that the agent abandons their authors. Often an agent will take their authors with them to the new agency, that is if the author wants to go with them.)