According to the episode 5, everybody was voting for Garrison because the memberberries were feeding them propoganda. We always see Randy go throught breakthroughs, like in the first season on how to avoid the volcanic rupture and one of my favourites was when he realised that Butters was feeding everyone his sperm. Likewise he was also able to deduce that memberberries can influence people's behaviour. How they do it, and who is behind is something we would have to wait and see. But maybe they influence people when they are ingested in that case Caitlyin got under control of the memberberries. Mike Pence, Caitlyin's real life counterpart is often regarded as superior to him and it can be said that some of his Trump's supporters are wooed by him only. So, my guess is that Caitlyin would now take a more active role in the election and help get more votes for Garrison. Obviously, Garrison would say things no sane candidate would say but Caitlyin would defend him (like Mike Pence does).
As for the behaviour of memberberries I think it was for comedic relief, they act adorable but their behaviour is akin to that of a mafia. Why they did it to one of their 'own'? Because that memberberry was severely tortured by Randy and was incapacitated .