Some random dude pulled up and is like I'm her dad and everyone believed him. This guy beat several guards and they stated earlier in the series that a skilled fighter can't beat magic so if that guy truly is her father there would be no reason as to why Merida can't use magic. So I call bullshit.
Well, everyone already suspected that Melida was not an Angel, so of course they’ll believe this guy. And the more she denies it, the more people will believe the accusations.