It's not really that uncommon for sellers to wait for news in any financial market. That's usually when the volume is and if they hold a large chunk of coins, that's the time with the most liquidity to exit.
I agree. However these dumps happen before a buy wall forms, and before sellers can benefit from some upward momentum.
It doesn't make sense to me. If I were trying to sell I'd put several asks in the path of an upward movement following the news, not dump it hard to neutralise all confidence.