I think she does it bc her mother,(wiki is worded weird)
WIKI
The new warrior crouched by the cliff shaking in her pelt. Her claws gripped the edge half afraid, half eager to fall. Her fur prickled as she imagined her clanmates' faces when they happened upon her broken body as they passed that way.
Then Nightwhisper remembered her mother. She'd probably spit on her corpse no matter how much shame she'd receive from her grieving warriors. This was all because of her. Nightwhisper thought as she attempted to launch herself into the stream. Her claws held fast.
Come on, Nightwhisper! You won't have a chance after this. This was the first time in ages her mother wasn't constantly watching her mistake of a kit. She was single-born, and single kits were a bad omen. So why don't you do your mom a favor? She watched the water speeding by with stars reflected in it. It reminded her of Sliverstar. Sliverstar was her best friend, and she gave her her warrior name she had waited much too long for. She spoke out against Bramblestar. She had made such a commitment, and now Nightwhisper was going to toss it all away.
She had loved only two cats in her life, but she loved them with the strength of the ocean. "Sliverstar, Breezesong." Nightwhisper had lost her only friends to the responsibility of leadership. "I'll miss you," she whispered to the stream. "You're the only ones who kept me going."
Nightwhisper released her grip and bit her paws until they bled so if she tried to hold on, she'd be too weak to save herself. "Goodbye, BrambleClan." She leapt forward hesitating on the last moment and struggling to pull herself back onto the cliff, but she lost all feeling in her paws and plummeted down where her body was broken and her soul was taken away instantly.
I hope they won't find me. were her last thoughts.