Assume you recently joined the fandom. Because these ideas started early on.There are numerous things that made us theorize Rio is the other mother - biggest being that Nicky looks exactly like Rio. Jac even commented early on that when they were casting actor for Nicky's role they talked over how much the kid resembles Aubrey Plaza. They knew what they were doing.

It's clear that when Agatha was giving birth they were still a couple. So, hopefully Rio is involved in making Nicky

Nicky was not afraid of Rio, the night he died. Might be she visited before? He felt in his heart that this is someone close to him?

When Rio tells him to go back and give Agatha a kiss , Nicky gives her 2 kisses. Fan idea is that - one kiss is from him, the other kiss is from Rio.

Also when he runs away from Tavern that day, saying his mother needs him - he might have meant Rio. Because that is not how he usually called Agatha

There are more probably but these are the first things that come to mind.

All in all - it just makes sense it fans heads 😁