I thing that Paige Hamilton had a very big problem with Anthrax. I saw an interview with Helmet and they asked about Frank and he went off about how Frank was mistreated and pretty much the whole thing of it was "Fuck Anthrax, he's mine now" sort of tone.
So I would imagine that;
A. Frank was not having that. Sure he left Anthrax because he needed space, but, Anthrax are his brothers and I'm sure he cares dearly for all those guys. I doubt he liked all Paige's shit talking.
B. Paige got offended at the Reunion idea. He had initialy told Frank he would like to have him on the next CD - but that was when Frank was right there. I'm sure once Frank's back was turned Paige said "all right now lets go find a new bass player."