... why did that batch of dragons attack Birk on the day of Valka's disappearance? They aren't the same ones that they've been at war with - those steal food for the "beta" in HTTYD's climax.

it related to the fact that Valka's dragons are disabled by Drago's traps?

The ones stealing the food in movie 1 were following a different "alpha" dragon, yeah. The ones who were taking food during that attack with Valka presumably stopped because of her.

I guess I must've been thinking the dragon raids were more frequent. Why did those dragons raid Birk? We're shown that their alpha feeds them well.

Maybe they weren't part of that group yet? There's a possibility that it was Valka who found the Alpha for her dragons, to help keep them safe.

But HTTYD establishes that they wouldn't attack humans without a reason.