Why did the gulf of tonkin incident happen?

Because North Vietnamese Navy (NVN) torpedo boats attacked the USN destroyer USS Maddox on 02 August 1964. Then, allegedly again on the 4th. The resolution occurred shortly afterwards.

The first attack did happen, it was in fact a sea battle between the NVN & USN.

The second attack may have been a mis-understanding coupled with a radar malfunction with one or more of the destroyers.