Chomsky's plausible explanation for Harvard Law School professor Alan Dershowitz's obsession is here:
Dershowitz readers will be aware that whenever his sensitive antennae pick up a phrase that might be critical of Israeli government policies, if my name is even remotely associated, it quickly becomes the “hard left gang of Israel bashers” led by the evil demon Chomsky. Why the consistent fabrications over the past 36 years – which, of course, merit no response? Dershowitz and I know very well, but others may be intrigued, so I might as well make the reason public for the first time. His pathetic behavior traces back to what was probably our first contact. In April 1973, Dershowitz wrote a scurrilous attack in the Boston Globe against Israel’s leading human rights activist, Dr. Israel Shahak, the chairman of Israel’s League for Human and Civil Rights, in which he even went so far as to support a government effort to destroy the League by methods so outrageous that they were at once declared illegal by the Israeli courts. I responded, correcting his slanders and fabrications – that is, every single substantive statement. He then tried to lie his way out of it, even descending to falsification of Israeli court records. I responded again, citing the actual court records and responding to his new lies and deceit.
The incident demonstrated conclusively that Dershowitz is not only a remarkable liar and slanderer, but also an extreme opponent of elementary civil rights....Dershowitz flew into a fury over the exposure, and ever since has produced a series of hysterical tirades and lies concerning some entity in his fantasy world named “Chomsky,” who lives on “planet Chomsky.” That is his standard style when he is exposed, reaching truly grotesque levels in his efforts to discredit Norman Finkelstein (and even his mother, probably a new low in depravity) after Finkelstein’s meticulous documentation of Dershowitz’s astonishing lies in his vulgar apologetics for Israeli crimes (Beyond Chutzpah).