This message indicates that the client has disassociated from the AP. When there are several APs in the vicinty, the clients may often disassociate with one AP and reassociate with another AP. This is because the clients are always scanning for better signal strengths and when they find another AP with a better signal strength they jump to that AP. To avoid this the design of the WLAN network must be reconsidered and probably a site survey should help.