Tasha may have come to be less than worshipful of Starfleet leadership and the Federation in general. Given more time, Tasha might have started to question why Starfleet wasn't sent by the Federation to end the overall situation on her colony world, a train of thought triggered by news of the death of her sister perhaps.
The Federation knew the conditions there. And unlike Bajor, the Federation had ready access to the planet itself. Tasha would have witnessed and heard of other examples of the Starfleet not helping people and entire civilizations when it could have.
Data: "... Starfleet has permitted several civilizations to fall. We have at times allowed the strong and violent to overcome the weak."
A disillusioned Tasha coming to identify with the Maquis cause, not impossible.