However there are two events that seem like are related in some way to this drop which are the launch of bakkt that was disappointment for many people as they were hoping that bakkt will push the price up upside and second one is hashrate which is still mysterious.
People are just connecting the dots, but what happened could only be answered by the one who dumped their Bitcoin.
No need to find what is the cause of this because that's technically not possible. You can debate it all day, but it won't give you valuable insights except for mere speculations.
#1 A new public key wouldn't defend against quantum brute forcing.
Then don't reuse your address. Other than that, move on to signature like Lamport signature.
Seriously, if SHA got broken then I guess banks or any mainstream public services that depend on encryption would take a bigger blow compared to Bitcoin.