Why Is a Sudden Drop in Hashrate a Major Security Concern for a PoW Coin?

A sudden drop in hashrate is a major security concern for a Proof-of-Work (PoW) coin because it drastically lowers the cost and difficulty of executing a 51% attack. The network's security is directly proportional to its hashrate.

When hashrate falls, an attacker needs to acquire significantly less computational power to achieve majority control, making a double-spend attack much more feasible and economically rational. This vulnerability is particularly acute for smaller coins that have less total hashrate to begin with.