A large pool can jump into a small coin, raise the difficulty level really high, and then jump out.  The remaining miners at that point have a hard time forming new blocks because the difficulty is so high. 

Recently a trend has started that some mulitpools will not only get a lot of coins while raising the difficulty, but then they will buy out the whole buy wall, in effect "dumping" what they earned.  These people then buy a POS coin that they are supporting.  It hurts the regular bitcoin and litecoin clones while helping the POS coins.  The POS coins can't be mined back so they aren't vulnerable.  Now while this trend has started, it is still very small and hasn't had an effect yet, but if it gets large enough, it will reach a tipping point and any miner that wants to make money will have to join in the multipools mining indirectly for POS coins.  So basically people that are pro POS could buy miners and then in theory ban together and kill POW coins.  It hasn't happened though.