Boromir suddenly realises that the girls thought Legolas more attractive than him, and sticks a few arrows into himself in a fit of agony.
OR
Boromir ALSO finds the
Children of Hurin trailer and knows that possibly even Boromir fanatics like MatthewM could think him 'not as cool as Turin'.
OR
Boromir just turns around and happens to look like that in a freeze-frame. Duh!
