The reportienter textng is Willis is pursuing a RICO case using the GA RICO law which is pretty broad and a conspiracy case. I'm not a lawyer, but I suspect that is the basis.

Lawyers who are described as Trump allies told Rolling Stone a lot of folks are getting indicted

Three sources who have spoken with prosecutors tell Rolling Stone that they believe that Fulton County district attorney Fani Willis is likely to indict not just Trump, but a number of his associates involved in attempting to overturn the election, as well.

“It really seems like they’re coming for everyone,” says one lawyer who has repeatedly dealt with the prosecutors in this criminal probe. “Based on what I know, Willis and her team do not seem to be stopping at Donald Trump. The scope for this [likely coming indictment] is probably going to be a hell of a lot wider than that…and round up a significant number of people.”