It's a bit different on Hive, basically you need 17 out of 20 witnesses to perform a hardfork, but I guess in a way you could also say a 51% attack if more than 50% of active stake were to vote for fake/overthrowing block producers which happened on Steem. A great article I can recommend going through the whole ordeal is this if you have the time, it's much better written than my post as well. ^^

https://decrypt.co/38050/steem-steemit-tron-justin-sun-cryptocurrency-war