Yeah it's definitely mixing scales as grimmdude mentions and he is bang on the money that diatonic is the way to start and learn concepts. That's when you start asking the exact questions you've just asked... Hey this chord progression doesn't fit my minor scale or major scale... WTF? Basically, you can borrow chords etc from other scales...
I personally interpret this as Minor throughout as indicated by what the guy who made it... But where I disagree with grimmdude is where he says it's borrowed from B Dorian... I think he's taken the E chord from the Melodic scale of Bm... Which would then be in keeping with his "Minor" but from a much more simpler point of view.
But... And this, unfortunately you see, is where (as Obi Wan once said) it's all down to a certain point of view.
@Grimmdude is right in what he said... I agree fundamentally and technically he is right.... however personally when explaining this I try to avoid 'modes' wherever possible because then this opens up the 'mode theory explanation' which does tend to confuse and bewilder because there is another set of theory behind these modes which then needs further explaining in theory terms... And sometimes it's just waay too much theory for the brain to handle... And sometimes there is an easier way...
So here's my simple interpretation;
Bm, A and G = Key of Bm natural minor scale
E = borrowed from Bm Melodic minor scale
Worry about modes as a separate theory subject... " or forever will it dominate your destiny "