According to a tweet from Bitfly (@etherchain_org), on August 1st 2020, there was a chain reorganization of 3698 blocks at block 10904146 on the Ethereum Classic (ETC) chain. This implies that there was about 3000 block-insertion by a miner who likely lost internet access while mining and the offending miner used the address 0x75d1e5477f1fdaad6e0e3d433ab69b08c482f14e, according to hackmd. This occurence is termed a 51% attack though, it didn't look outrightly malicious.
Talking of chain reorganization or chain split, it occurs when any two forms of blockchain compete for legitimacy from hubs in the system. Similarly, a 51% attack is a potential attack on a blockchain, where an entity has majority of the hash rate, thereby disrupting the network of the blockchain.
Because of this attack and disruption on the Ethereum Classic chain, exchanges and individuals were urged to pause ETC transactions such as depositing and withdrawals because of the network's instability as at that time. It also caused all nodes to cease synchronization. But it was recommended that miners continued mining.
Source: Binance Chart
So far in the market, ETC seems unaffected by this news as it is still performing and joining BTC and other Alts in the pumping rally. ETC recently hit $8.347 and then dropped to $6.316 but presently, it is trying really hard to maintaining a support of $6.982 on binance futures.