Early on, the operator (and also one of the bitcoin devs) of one of the biggest pools secretly diverted the pool's power (a lot) to at least one other sha-256 alt-coin to kill it.

A couple months back the now biggest pool (part of big mining scam to consumers if you ask me) was proven to have used it's majority power of the network to construct and successfully execute double-spendings. They only needed their 29% majority share of the network (and a less-than-secure victim). They now have 33%, no one is doing anything, the news doesn't get out and people keep buying GH at insane prices from them and giving them control over the network.