i dont get how mona died

She fell off a cliff and died when she and Spencer were fighting after the girls found out Mona was A.

Hanna knew that Mona was A, but forgot when she was hit द्वारा the car. She remembered when Spencer and Mona were in a car going to the police station. So Hanna texted Spencer, but Mona figured out that Spencer knew (she was all, "uh, I gotta go!") so Mona tried to strangle Spencer, but it didn't work and Mona ended up falling off the cliff.
Kinda confusing, I know!