Thanks for the replies. There are some good points I hadn't considered already.
Basically, the motivation behind the Typhon Pact was that Bacco browbeat these nations into working together with their Federation rivals for the greater good, and it worked, and so they got to thinking, "Hey, this working-together-for-mutual-benefit thing works pretty well... but do we really need to include the Federation? If it worked for them, why can't it work for us independently of them, and make us collectively strong enough that they can't push us around anymore?"
But why all join up to side against them
(the Federation)? The Feds were the ones who planted the idea of joining up in the first place. Why not join against the Tholians or the Breen? I find it very curious how a group such as the Romulan Star Empire who fought against the Dominion and suffered severe losses would so readily form an alliance with a group like the Breen who who sided with the Dominion. Storywise, it will be interesting to see a further rift growing between the IRS and RSE... but it does seem to stretch credibility a tad to suddenly be sharing currency with the Breen. I know people have pointed out that many real world nations forged alliances following wars with one another, but A Singular Destiny, pretty well the sole purpose of the Typhon Pact seems to be to serve as a thorn in the Federation's side (messing with their refugee locations, etc.). Why would the RSE join the Pact when they seem to have many more reasons to be against the Breen than against the Federation?