I watched the movie (English-dubbed). The movie gives no explanation of why Pikachu is suddenly shown speaking a human tongue.
However - and this is just my impression - I don't think that Pikachu actually did speak a human tongue here. The astute Pokémon-watcher will recall that there are many scenes in the anime where a Pokémon is able to convey complex thoughts to its trainer just by making its species' noises. This happens particularly often with Pikachu/Ash.
So this scene felt to me more like a "translation convention" for the benefit of the audience, who, unlike Ash, cannot understand what "pika pi pikachuuuu pika" means. Typically, when the audience needs to be apprised of what Pikachu is saying, Ash will reiterate what Pikachu supposedly said to him (you know, along the lines of "What's that, Pikachu? Team Rocket's giant Meowth-shaped balloon is over thataways?"). But I think that approach wouldn't have worked very well in this particular scene, so we get Pikachu "speaking" a human tongue instead. This works fine here, because the only character who witnesses Pikachu's "speech" is Ash, who would understand Pikachu just as well even if Pikachu were speaking Pikachu-ese.
As supporting evidence, I note that Ash doesn't seem even mildly surprised by this turn of events, which suggests to me that Pikachu was still speaking Pikachu-ese at him. (Though, I suppose, he might have had other things to worry about, like the gang of mind-controlled Pokémon that are about to beam-spam him out of existence.)