More details are listed here
https://blog.ethereum.org/2019/01/15/security-alert-ethereum-constantinople-postponement/Basically its one of their EIP that has this issue and they will most likely remove it from the constantinople upgrade. It had something to do with gas prices and it looked like a minor issue but they decided to postpone it just in case since its an important update.
This delay really isn't an issue however it caused panic anyways as you can see by the way the sell-off occured. Basically happened moments after the ETH dev on twitter announced the delay. Most likely it was due to the fact that the fork was less than 2 days away. Maybe if it was a month away, it wouldn't of caused such a great stir.