For those out of the loop, miners work together in pools to combine their mining power. With more mining power, the more likely it is for them to mine a block (of the blockchain) and divide its payout among members of the pool. Pools cause centralization of mining power. You can see a list of pools and the percentage of their mining power at [https://blockchain.info/pools].

Looking at the chart, you can see that the top 5 mining pools can collude in being a single entity to "control" the blockchain (I believe this is called a Sybil attack [https://en.wikipedia.org/wiki/Sybil_attack]). Now keeping incentives in mind, the idea is that people using bitcoin have faith in the system. That gives bitcoins value. Miners get paid in bitcoin. If they were to manipulate the blockchain, that would most likely cause bitcoin's userbase to lose faith in the system and thus what the miners would be stealing would suddenly have little to no value.

I'm not positive, but I recall hearing that pools have been intentionally been made smaller to keep users' faith in the system. Perhaps the goal of these majority mining power is not to steal bitcoin but censor certain users? That may be a bit more complicated depending on the users' opinions on a case-by-case basis. Fun stuff :)