I think it was sheer coincidence that he caught them when he did, but I think it stewed in his mind that she went with Bolin just to bother him. And it did. So he planned on confronting Korra later, but later turned into NOW. Bolin walking away and giving him the chance to talk to Korra alone was an opportunity.

Maybe in some way Korra did feel like Asami was 'intruding' on their team, but I don't think she ever meant to come off as rude as she did. She NEVER brought anything up to Asami, and probably never would have if it wasn't for Bolin saying something in canon. Mako is just as guilty. But seeing as this is AU and we have time to explore, I'll need to figure that part out soon.

Lol FUCK TANNO, he was everything Amon resented in benders anyway.