THE GEMORRA SAYS ANYONE WHO IS PORESH FRM THE TZIBBUR THEN 2 MALACHIM PLACE THEIR HANDS ON HIS HEAD AND SAY..... IS THERE ANY SIGNIFICANCE TO THEIR PLACING THEIR HANDS ON HIS HEAD?? THE GEMORA COULD HAVE LEFT THAT PART OUT.
We find a similar expression in Shabbos (119a). There, the BEN YEHOYADA (in SEFER BENAYAHU) explains what it means based on Kabbalastic principles. The simple understanding, though, is that the placing of their hands on his head is a way of emphasizing the severity of what happens -- it is not just words that they speak about the person, but they do an action, which emphasizes the point that they are making.
Moreover, the placing of the hands upon the head is like making the statement that, "His punishment is on his own head ("Damav b'Rosho")! It was your own actions that are the cause of what will befall you!" (The same applies to the expression as used in Maseches Shabbos, which Rav Ben-Arza mentioned.)