Why does Lennie panic, and what happens a result of his panic? How does this similar to an event earlier in the story?

I assume you mean prior to accidently strangling Curley's wife? Lennie panics when Curley's wife struggles because his "petting" of her hair is too rough. When she begins to scream he shakes her.... breaking her neck. This scene is reminiscent of the dead mouse.... and the woman who cried rape.