Just an aerodynamic explanation (a guess):
The lift over the wing is created by a vacuum (negative pressure distrubution). This vacuum will suck the unlocked spoilers up and cause them to be floating. Eventually the created lift somehow will be destroyed. To maintain the desired lift, the AOA (during flare this can be commented as body attitude) should be increasead and consequently this may cause a tail strike.
The shape of the pressure distrubution ( vacuum) over the wing surface will be varying with AOA. Higher AOA at Flap 15 will make the the peak point of this vacuum shape close to leading edge as reducing the likelihood of this floating and so the tail strike as well.