That was the small-minded “folkies” who were booing, they believed Dylan was a sellout because he switched to the electric guitar.
On one of the recordings of Dylan & The Band’s early live shows in England, you can hear one of the folkies yell ‘traitor’, you can hear Dylan tell The Band, ‘play it f***ing loud’, and they kick into ‘Like A Rolling Stone’.
But remember, folk was the music of protest, it was the music of intellectuals. Dylan was viewed as the best hope to take up the torch carried by lefties like Woody Guthrie, Pete Seger, etc. Rock was just viewed as disposable pop music drowned out by audiences of screaming teenybopper girls. Seeing Dylan appearing onstage with what would become The Band, with electric guitars and drums no less, was like watching The Devil appear.