I've parted with a couple of agents. I left the first after five or six years simply because I wasn't particularly happy with her knowledge of teh pubishing industry outside a specific genre, and I received a better offer with a better known agent and a larger publisher.
This second agent wasn't right for many of my projects, and we agreed up front that it would be a short term arrangement, so I didn't really leave her.
I left the the next agent after a few months simply because we did not see things eye to eye. She thought she was in charge, and should decide which books I wrote for who, and she wanted to critique each one.
I thought I was in charge, wanted to write the books I wanted to write, and believed I knew more about actual writing than an agent. This termination was not particularly amicable, but not particularly vicious, either. I simply lauighed at her demands, told her we were through, and hung up.