Steemit Inc. did not freeze the blockchain. Hive account@nijeah did. Hive account@nijeah tried to power down a negative amount of Steem Power, which the code did not check for. This lead to various other values, like total supply values to be wrong, which caused errors in witness nodes. These errors caused them to stop producing blocks, effectively freezing the blockchain.

Then, once Steemit Inc. noticed that the chain was frozen, and witness nodes were having issues, they released a hard fork update that

• Undid the negative power down
• Stopped new negative power downs

This patch was quickly peer-reviewed and merged. Then they urged witnesses to update, so they could continue producing blocks on the new chain. The hard fork didn’t pass through any formal committees though. Witnesses reviewed the patch, and decided to update. (Became it was a good idea, and because it was the chain that Steemit.com was using).