A321, idle thrust, F speed & F3 hand flown turn - those are the usual culprits. Just to make sure, the warning is by no way tied to undershoot the commanded speed. FCOM1 goes to say (my recollection) that is triggered when energy state is such that next pitch up command would not result in change of trajectory unless thrust is added.
As a bit of creep, does anybody know how is alpha max calculated? I suppose even on a FBW, it is not at aplha cy max, as any upset would put you on the downslope of aplha/cy.
FD (the un-real)