User blog comment:Kogath/Monster Hunter 4 Ultimate for the Nintendo 3DS/@comment-17735225-20150319164455

Apparently, the quest where you fought rusted kushala in battlequaters(g-rank version called operation rust remover) is unlocked by either failing rusted kushala quest(wind of discord) or repel him.

The one that invades the city is the very same one that you failed to kill/repelled before. This is confirmed when one of my friends unlocks it and notices that it has the same breaks as the one he repelled in the last quest(tail already cut, wings broken). Not too sure if it keeps the hp or not though because we don't know how well he did the first round; it dies relatively fast anyway. It also explain the weird clawed mark on the quest info background, it's a revenge quest.

After you beat the first one it'll change back to normal occasional urgent quest, and contains fresh rusted kushal.

Talk about being vengeful. I wonder if this revenge rule applies to other elder dragons as well.

The one in the story also appears to only target Ace commander at first too.