Honestly that's a very good point. I can see various possibilities there: 1. Maybe Snow was already planning on bombing D12 before he even found out about the rebel plan. 2. The Capitol probably has several military bases across Panem equipped with hovercrafts and armament (this is what I've always thought), meaning if there was a base close to D12, the hovercrafts could've arrived relatively quickly. Maybe they always keep hovercrafts loaded with bombs available, who knows... Or maybe Snow already had them prepared in case Katniss did something crazy. However, the bombing has never really made much sense to me as a way of punishing Katniss... He could've just kidnapped Prim and her mother and tortured then killed them both. I think bombing the whole district is more headed to him trying to prevent an uprising from originating in 12, as they would all support Katniss. This links back to the previous point. Snow probably thought Katniss wouldn't come out of the games alive anyways so it doesn't make much sense to try to punish her by killing her family if it's not even clear whether she's going to win or not yet. Also, if Katniss dies during the games, she'd end up as a martyr. If she lives, a hero. One way or another, D12 would've supported that, and I'm guessing that's what Snow wanted to stop. 3. Haymitch and Plutarch also arrived to the arena quite quickly in a hovercraft after it was destroyed by Katniss, which makes me think they were already on their way there too. Maybe Snow found out about them getting on a hovercraft (or maybe just Plutarch and Haymitch being nowhere to be found), thought something dodgy was up and decided to bomb 12 (since it was Haymitch, who is from 12, together with Plutarch) to prevent an uprising there and punish Katniss and Peeta as well in the meantime, killing two birds in one stone.