The Nothing-at-Stake problem is a security & consensus vulnerability that is hypothesized to plague Proof-of-Stake (PoS) cryptocurrency networks in events of forks.
While in the event of a fork in a PoW blockchain the consensus is maintained through the allocation of computational power; however since in fork vents participants are distributed the new chain’s assets for free; when a fork happens in a PoS network, due to the asset distribution, the network validators are able to retain control over both networks evenly and for the newly forked child-chain, the validators have no “skin-in-the-game” and thus no reason to act fairly.