I think the information is either incomplete or someone has gathered a fact and reported it without understanding what it means.
My understanding is that the wing/tail feather mode is normally used only during re-entry and perhaps M 1.4 is the Mach number at which is is normally deployed.
Dynamic pressure is greatest at transonic flight (Max Q), then reduces as the aircraft goes supersonic and accelerates past Mach 1.2.
So for safety, the feathering mechanism must be
locked when flying through this region. Otherwise, the aerodynamic forces can twist SS2 into feather mode.
Which is what apparently happened.