Filming begins in May 2016 and the season will air spring of 2017 in the UK.
Everything we know so far about âDoctor Whoâ Season 10 - CultBox
Actually those seasons aired the same time over here too. They do it to avoid spoilers the best they can.
I distinctly remember getting an issue of Entertainment Weekly talking about how Clara is leaving the show several episodes before it even aired here in the US. They knew this only because the season had already aired in the UK. It was a total spoiler for people who hadn't even gotten that far into the season.
That's lame that they announced it. Maybe it was an attempt to soften the blow for Clara lovers.
I think a paper or magazine over there broke the news about it and besides the BBC never keeps stuff secret, they like the publicity from it. Moffat wanted Matt Smiths leaving and regeneration to be a surprise but the BBC would have none of it.