Ahsoka Tano faked her death at the end of Star Wars: The Clone Wars for more reasons than one. By the final episode of the series, the state of the galaxy is worse off than it was during the Clone Wars. Darth Sidious came out from the shadows, overthrowing the Republic and establishing the Galactic Empire in its stead. He also called for the destruction of the Jedi and their supporters, which immediately put a target on Ahsoka's back. She had to make a daring choice to save her life and begin a new one under his rule.
Both she and Commander Rex miraculously survive the initial command of Order 66, which compels all clones to kill their Jedi generals. Thanks to Darth Maul, their Venator-class Star Destroyer crash lands onto a nearby moon. But unlike the clones, Ahsoka and Rex escape just in time via a Y-Wing bomber, safely landing on the surface. Once there, they bury their fallen comrades and take a moment to reflect on their past life. Ahsoka leaves her lightsaber behind as a marker of her death, signifying to the Empire that she was killed during the Jedi purge.
There are a handful of reasons for Ahsoka's decision to fake her death, the most obvious being that Order 66 remains in effect for years to come. The Jedi were public enemies as long as the Empire maintained control of the galaxy. Darth Vader himself was tasked with hunting down the survivors and eliminating them, as the final scene of The Clone Wars depicted. Thus, Ahsoka faking her death is a genius way to keep the Sith Lord and his troops off of her trail. If they believed she was no longer a threat, they wouldn't go after her.
Though it's the most understandable, Order 66 isn't the only rationale for Ahsoka faking her death. She isn't looking for a simple life of exile; she quickly wants to do something about the Empire and their oppression. She works closely with Senator Bail Organa and his Rebellion years after the events of The Clone Wars concluded. Still, few of her fellow operatives know her true identity, as she adopts the storied pseudonym "Fulcrum". She had no choice but to reinvent herself if she wanted to stay alive in the new era. She could still make a difference in the galaxy without the life-threatening baggage of her Jedi past.
The Clone Wars finished out an illustrious 7 season run on quite a down note, dissolving both the Jedi Order and the Republic. So begins the reign of Vader and Sidious' Empire, a regime created to instill fear and hoard power. Instead of facing them head-on, Ahsoka chose a more low-key approach, undermining their efforts in secret. Faking her death after the Siege of Mandalore is a survival tactic and a form of reincarnation. She became a new person, distancing herself from the Jedi and becoming a full-fledged member of the Rebellion. Ahsoka, as seen in The Clone Wars, is gone, but Ahsoka, as seen in Star Wars Rebels, is just getting started.
