Moses surely would have told Zipporah about God's command to circumcise. While we have to speculate about the details, the Pulpit Commentary suggests a plausible sequence of events.
The suggestion is that it was the eighth day after the birth of their second son; that the first son had already been circumcised; and that Zipporah talked Moses into not circumcising the second.
The narrative of verses 24-26 is obscure from its brevity; but the most probable explanation of the circumstances is, that Zipporah had been delivered of her second son, Eliezer, some few days before she set out on the journey to Egypt. Childbirth, it must be remembered, in the East does not incapacitate a person from exertion for more than a day or two. On the journey, the eighth day from the birth of the child arrived, and his circumcision ought to have taken place; but Zipporah had a repugnance to the rite, and deferred it, Moses weakly consenting to the illegality.
The commentary goes on to suggest Moses may have become seriously ill, and recognized it was a judgment for his weakness. He no doubt would have said something to Zipporah, who then might have needed to do the circumcision because of Moses' weakness, but complained about it. According to the commentary, the complaint likely referred to both circumcisions.
The commentary acknowledges that the language suggests a miraculous appearance, but that it could refer to an illness as well.