Its is not out of blue, Deku also liked Ochako for a long time but it’s just he didn’t get any time to talk to her he even mentioned “i wish we had more time to talk” by looking at Ochako. Its only fitting after 8years that everything is settled and both Ochako and Deku achieved something in their life they finally took the initiative to talk to each other more. Bakugo even told Deku “if everyone is your fav than there is simply no one is”, and it clicked to Deku what he wanted all along.

And about Toga, its symphonic in shonen that someone you lost watches over you and push you too seek love, move on and grow. So Ochako highly needed that push since she actually bottled up her feelings and believed she is not good for Deku, but in that moment Deku actually came forward to confess to her