Sinema is, at heart, a liberal Democrat that is restrained from voting her true progressive sentiments by a voting population that is not wildly progressive. She made the political calculation that Independents in Arizona are a larger registrant constituency than either Democrats or Republicans. She gets to keep her Dem constituency and can pick up Republicans who are not averse to voting for a Dem with an I next to her name on the ballot as well as the large Independent constituency here.
It may be she recognizes the authoritarian/totalitarian movement in her own party, and does not want that system to triumph.
She is smart enough to understand the threat.
She has said before that the USA should reap the benefits of both capitalism and leftist regulation...
There are probably several motivations for her switch, but this may be one of them.
I do think this is part of her thinking. At one point there was talk of her running for President. Maybe she is ramping that up to challenge Joe or run in his place if he doesn’t.