No activity for 3 min and the client did not drop the connection ? Reason code 4 (https://www.cisco.com/assets/sol/sb/WAP371_Emulators/WAP371_Emulator_v1-0-1-5/help/Apx_ReasonCodes2.html)
http://forum.mikrotik.com/t/meaning-of-disconnected-received-deauth-no-activity-4/137874/1
An AP will hold a frame to be transmitted for ever. (It will not drop that frame unless disconnected). Could this hold up a queue ???
There is a “Frame Lifetime” in advanced tab of wireless interface, to drop old frames. Default is zero, never.