Apparently, it was not actually a hard fork. It was a bug that made old nodes not follow any alternative blockchain, instead they just seem to stop and refuse follow any chain.
You can read more about it in these links:

https://forum.z.cash/t/recent-chain-fork-not-actually-a-chain-fork/14172/3 http://bitcoinist.com/zcash-suffers-chain-fork-malicious-act/