Quote from noflynomore:
"That little area of ice at the base of the stab is were there is a big metal spar right under the skin where the fin attached to the fuselage."
There's a small area on the top of each A320 wing that does the same thing. IIRC, it's where the rear beam of the landing-gear bay intersects the rear spar (i.e., the apex of the so-called Bermuda triangle). Some engineers on the ramp have a hand-held de-icing bottle to melt it.