I am guessing that the key pair generation process was faulty. The FBI found an exploit in a wallet used by the hackers allowing the private key to be predicted. The prefix is bc1,which is uncommon. A few weeks ago there was such a vulnerability with Cake Wallet.

Or they installed malware on the hacker's computers and were able to log the private key as it was generated.

Or the hackers foolishly stored the key pairs on a server

Bitcoin is falling and this news does not help because it shows that some aspect is less secure than previously thought.