UKIP Twitterers have been accusing Carswell of duplicity for ages. Farage and Arron Banks hate him and have been trying to oust him. He upset Farage by not accepting a considerable amoyt of money from the EU (can't remember ecatly how much it was - about a £million?) for administrative expenses. Carswell's argument was that he didn't need it, because he was the only UKIP MP, so turned it down. Farage was furious.
Carswell is a strange character and I don't really understand why he defected to UKIP. He certainly doesn't share their narrow-minded ideals, especially over immigration. He's an ultra-libertarian, which includes having complete freedom of movement for workers. He's against the EU, because he sees it as restricting total free trade. He's generally against workers' rights and would like to see the UK turned into a bigger version of Singapore. He couldn't care less about the 'left behinders' in globalisation, who are the voters UKIP tends to attract.