What is "If ghash.io hits 51% people would just leave them?" referring to? I am watching video that explains original Bitcoin whitepaper by Satoshi Nakomoto.

I totally understand that as long as 51% of the CPU power is legitimate then using bitcoins is safe.

But then the person says, "If ghash.io hits 51% [hashing power], people would just leave them".

I listened to this part so many times, but don't understand what it means.

Does it mean if one entity has over 51% CPU power, the network would disconnect them?
No, the network wouldn't disconnect them.  There's no way for that to happen.

Recall that a pool is an association of miners who agree to share the proceeds of their mining; whenever anyone mines a block, the other members of the pool get a share.  But miners are free to leave at any time, simply by stopping mining blocks for that pool and joining another one, or switching to solo mining.  

There is a general belief that it is not good for the Bitcoin community for any one miner or pool to control more than 50% of all mining capacity.  The speaker in that video believes that if ghash.io miners were to eventually constitute 51% of all mining power, then some of those miners would choose to leave the pool.  It could be argued that it would be in their self-interest to do so; they'll make approximately the same mining profits no matter which pool they use, but since their profits are in Bitcoin, they have an incentive to ensure that the currency stays stable and usable.  

The sentence you quote might sound like a statement of fact, but it's really just the speaker's prediction as to what he thinks would happen in that situation.  He might be wrong, but many other people have made the same prediction.