There is a pattern that Valve manage their case drop in the active drop pool, usually it's following the two rules 1) first in first out 2) or lowest key sales out, and operation cases drop are exclusive to pass holders then after the operation ends, they become a common drop for everyone in the game. The only operation cases so far in the history of CS:GO that become a rare drop after operation ended is the Vanguard case and the Hydra case, the reason why Vanguard case became a rare drop immediately after the end of operation, it was speculated by the investing community that it was due to its lowest key sales by the end of operation, so Valve moved it to rare drop pool immediately.
This time with Hydra case being moved to rare drop pool immediately is partly due to low key sales and low average key price, we still cannot confirm the rule that as long as the case is a quest drop, then the case will become rare drop after operation ended, you can't confirm that. The decision to make it common drop or rare drop, it is totally up to Valve.
Now, the fact that the hydra case became a rare drop immediately is a huge surprise among the investing community, even though we have figured it out as fast as we could after the patch, the general consensus is that the hydra case will become a common drop due to how Valve manage their case drop in the history, Vanguard was really exceptional and no one was expecting it to happen ever again, plus CS:GO has been doing horribly for the past 2 years where Valve makes everything easily accessible for everyone, there is no longer any kind of collector's tier item anymore like Katowice 2014, AK fire serpent, AK case harden, etc. Traders lose interest in new stuff because of how flooded new items are.
And the reason why hydra case go up rapidly and wild, we have all discussed with them long ago, and we don't want to mention them again but the following threads are the summary and proof.
First awareness of the hydra case being in the rare drop pool
Proof