I believe it's initiated by mesh system. Here's a sample log entry from my aimesh setup.
Jan 30 06:54:35 roamast: discover candidate node [D8:50:E6:CF:84:E8](rssi: -55dbm) for weak signal strength client [2C:33:61:AE:7C:5A](rssi: -73dbm)
Jan 30 06:54:35 roamast: eth2: disconnect weak signal strength station [2c:33:61:ae:7c:5a]
Jan 30 06:54:35 roamast: eth2: remove client [2c:33:61:ae:7c:5a] from monitor list
84:E8 is my node and 7C:5A is the client. It would appear the environment understands there's better connectivity to the node so it actively disconnects the client from the router. Note this is not an enterprise solution where both the AP and client work together for a true seamless hand off. In this case it's a hard disconnect, with the idea being that the wireless client will attempt to reconnect to better signal available on the node.
At least that's how i understand it!
Hope that helps.