Hanna knew that Mona was A, but forgot when she was hit by 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!
