“The attackers, calling themselves the ‘51 crew’ are overpowering small blockchains with more hashing power and by DDoSing other nodes. It seems as though the attackers are specifically targeting Ethereum-based blockchains as the cryptocurrencies, Krypton (KR) and Shift, both Ethereum type coins, were threatened with a hack if they didn’t pay a ransom to the attackers.”

Source: Group of Hackers Are Holding Small Blockchains Hostage by 51% Attacking Them – Bitcoinist.net