Ethereum is undergoing a chain-split, after canceling its planned upgrade last minute. Currently about 10% of Ethereum miners are splitting off with the herd, going ahead with the Constantinople upgrade which was called off.
In a Reddit comment, Michael Hahn of MyCrypto noted that the split was actually an expected outcome, with the upgrade being called off only 30 hours before it was planned to take place, which didn’t leave enough time for the dev team to contact every mining entity.
Currently, based on hash rate, the estimate is that 10% of the miners have erroneously went ahead mining on the upgraded chain. Trust Nodes recommends that users avoid transacting on the Ethereum chain until the issue is settled, but advises that this is likely not an emergency situation for the cryptotoken.
The most likely cause is simply that it takes a number of days for information to spread between major languages, and the dev team simply did not have enough time to get out the information to everyone before the chain split.
The accidental fork shows one of the drawbacks of having cryptocurrency authenticated through a decentralized mechanism, and the ability for any contingency of miners to suddenly change directions is baked into the fundamental rules of cryptocurrency.