We all know what supported the arrests because it's the same thing that always happens. Politicians felt there was a name to make for themselves by appealing to an unhinged and illogical mob against all sensible justice. Remember when Stephanie Rawlings-Blake thought she could make herself a national name. And especially Marilyn Mosby.
I don't know that. I assume that probable cause was found since they have been charged, and I'm willing to keep an open mind that there will not be proof beyond a reasonable doubt of their guilt.
To be fair this case is more clear cut than many previous BLM cases, and Officers Keung and Lang were holding Floyd down, so if Chauvin committed a crime then they certainly helped in it. And Thao did as well by physically protecting Chauvin from bystanders. You could say they were accessories to a crime which warrants being charged, in a similar way that the getaway driver is charged even though he didn't commit the robbery.
As far as politicians "making a name" sure, but absent public pressure the police and prosecutorial systems have an incestuous relationship. What's really needed is a truly independent prosecutorial branch that oversees the police, but we don't have that. Without these cell phone videos the whole thing would have been swept under the rug.