ANSWER:

List of reason codes(ref):

0...noReasonCode...Normal operation.
1...unspecifiedReason...Client associated but no longer authorized.
2...previousAuthNotValid...Client associated but not authorized.
3...deauthenticationLeaving...The access point went offline, deauthenticating the client.
4...disassociationDueToInactivity...Client session timeout exceeded.
5...disassociationAPBusy...The access point is busy, performing load balancing, for example.
6...class2FrameFromNonAuthStation...Client attempted to transfer data before it was authenticated.
7...class2FrameFromNonAssStation...Client attempted to transfer data before it was associated.
8...disassociationStaHasLeft...Operating System moved the client to another access point using non-aggressive load balancing.
9...staReqAssociationWithoutAuth...Client not authorized yet, still attempting to associate with an access point.
99...missingReasonCode...Client momentarily in an unknown state.

Might be this “Won’t fix” bug. I get from that report that NetworkManager is having trouble determining which one is “best”.

Disclaimer – I use wicd, and it seems to work in your scenario.

Call me V from http://askubuntu.com/questions/298451