The deployment task for the agent is not version-specific, as you can only have one agent in any given branch - so if you have a deployment task set to install the agent (as opposed to "agent version $foo") then this would cause the machines to upgrade the agent if a newer version was checked in. I don't know if that matches what you're seeing?
Thanks for the reply Joe.
I have just found agent update task causing the update. The other installed it were picked up by Lost and Found.
That's a bit odd - agents should only be upgraded by a deployment task, not an update task...
As long as you've got to the bottom of it