...Also I thought the risk of any kind of 51% attack was mitigated by POS? They would require a significant amount of the available coins (stake weight) in addition to controlling 51% of the network. Am I wrong?
POS blocks are not always found between POW blocks. Consider the consecutive series of blocks
571032,
571033, and
571034. All three were found a few seconds apart and by the same address (dwarfpool's JdaWQx5PCseQUB96FUZ4vALkfPYFUzVG6U). That event 10 minutes ago all by itself is already flirting with the possibility of a fork.
Looks like miners who care about the coin should be aware of this.