Frank was particularly angry over Cristian Romero's injury-time dismissal.
Skipper Romero received a second yellow card for kicking out at Ibrahima Konate. Xavi Simons was also sent off in the first-half for clattering into Reds skipper Virgil van Dijk.
"There is a massive mistake by John (Brooks) on the pitch. Two hands on the back by (Hugo) Ekitike," said Frank.
"I don't understand how he didn't see it. OK. Luckily we have VAR so they will bail you out when you need it, which they didn't. That was the second mistake.
"I think if you go back to that (second Liverpool goal) and the referee did his job, then I think it would not have been a first yellow (for Romero). Is that fair to say?
"I have a player that is extremely passionate, and if you have a passionate player then you need to go to the line sometimes."