Years ago, GHash.io had claim of 40% of the network hashpower. They were close to 51%, where they would have the power to initiate a 51% attack. Now it has 0.2%. How did this happen?
Apparently they were hit with a massive denial of service attack in response to their growing hash rate. That combined with a subsequent drop in the price lead them to stop their cloud mining operation and focus on their exchange (cex.io).
The other non cloud-miners who left the pool most likely did it in response to the massive appeal from the community to find another pool.
In general, miners will quit a pool when it gets too close to 50%, because they know they could cause the network a lot of trouble if a 51% attack was ever pulled off.
Not sure of the specifics in this case, but in general that's the built-in safety mechanism.