What is the nature of Lennie's attack on Curley's wife?

Panic.

Like at the start with the girl and the red dress, when Lennie touches something and the person with it starts flailing, Lennie panics, and just clings onto it, never letting go. This is the same with Curley's Wife's hair. As for breaking her neck, I think he was trying to get her to stop screaming, and he tried too hard.