An official incident report from Impermax Finance stated that a hacker was able to steal approximately 9M IMX from several wallets controlled by the team. The IMX was not sold immediately after the hackers stole the funds. So the official team decided to get a head start by dumping a lot of tokens on the market before the hackers did anything. The Impermax lending protocol is completely immune to this, as the attack is caused by stolen private keys, not a bug in the smart contract.