Conversation so far:
Who said Kenneth, what is the frequency?“
In 1997, a TV critic writing in the New York Daily News solved the mystery, publishing a photo of the alleged assailant, William Tager.
Who is william tager
On October 4, 1986, while walking along Park Avenue to his Manhattan apartment, Dan Rather was attacked from behind by William Tager, demanding to know Kenneth, what is the frequency?“
Why did William Tager attack Dan Rather
Tager claimed he thought television networks were beaming signals into his brain.
Why did William Tager say, Kenneth, what is the frequency?.
Tager tried to force his way into an NBC studio with a weapon to find out the frequency the networks were using to attack him and block it.
Was Dan Rather hurt when William Tager assaulted him?
As William Tager pummeled and kicked Dan Rather, he kept repeating the question, Kenneth, what is the frequency?“
Did William Tager serve time for the attack on Dan Rather?
William Tager received a 12½-to-25-year prison sentence for killing NBC stagehand Campbell Montgomery outside The Today Show studio in 1994.

Question: why did he think tvs were beaming signal into his brain.
Self-contained rewrite: why did William Tager think tvs were beaming signal into his brain.