Hello Bianca,
Something very similar is happening to us here in Lisbon, Portugal where we live and declare taxes and have 2 listings. "Out of the blue" we pay now taxes in the US: we have apparently already paid several hundred € to the US tax authorities and will pay taxes here in Portugal too (since it's here we declare and pay taxes).
The fact that we submit airbnb every year a withholding tax form (RFI-21) doesn't make a difference since we are now double tax payers and pay around 53% in taxes (US and Portugal combined).
We really don't know what to do. We can fill in that form (W8BEN) airbnb wants us to submit without knowing why and on what grounds and without any rational explanation from airbnb.
Could you give us some input? Many thanks!
Nicolas & Maria Carlos