Why would a miner, that is mining the large majority of a coin, fork or even control more than 50%? So they can make the coin they are trying to get worthless?
its not miners who believe in the coin that would attack it.
miners who are mining other coins... or hackers who can attack and then control one of the pools would temporarily move their power over to the Nautilus network to gain 51%+ of the hashing power.
from there it allows them to create a fork and control the block production (so they can make transactions and gain all newly minted coins while they have control) they then shift those coins onto an exchange.
once on the exchange the attackers can sell those coins for BTC or another coin and then withdraw those coins..
so within a day or two they can dump a shitload of invalid coins into the system and then leave everyone else to clean up the mess and pay the bill..
once the mess is cleaned up.. the attackers can then try again and again and again. until either the exchange collapses or the community loses faith in the coin and abandons it.
while the 51% attack is happening.. ALL other transactions on the network are invalidated so anyone else using the network is also screwed.