"Troll" was a B-rated movie that came out a good decade before Rowling first published a "Harry Potter" novel. For those not familiar with the movie, it stared a character names Harry Potter Jr. that featured magic, wizards, and a villain possessing someone close to him. Potter Jr. teams up with a witch in order to save the world from an evil troll.
While those are similarities, it seems more like inspiration more than anything. Media influences media. How many different versions of the same story have you read or watched in film. The filmmakers say that they didn't prosecute the matter when Rowling first published her books because of "family reasons."
They are only coming out about it now because "Troll: The Rise of Harry Potter Jr." is being scheduled to be a new animated remake of the film. They've already casted Patricia Arquette for the 2017 film. While Rowling and Warner Bros. are claiming the accusations are ludicrous, it seems the filmmakers just want to be clear that they aren't stealing from Rowling.