Let's face it it's about one thing: making money. Once a chain has the majority of hash rate other miners will switch to preserve their investment. Why you ask? Bitcoin is all about security and that security is preserved by having the majority hash rate. Once a fork starts the minority bitcoin chain developers have to make a decision. Do they go on with a coin that is less secure and can be destroyed by the majority chain at any time, change the POW algorithm or try for POS. Sticking with a minority hash rate POW is dangerous and you can't have a secure coin with it. That means change the algorithm before the majority can attack and destroy your chain by mining empty blocks.

Lets look at something as low as a 60/40 split. That is one chain with 20% more hash power than the other. Lets take half of that and attack the other chain. So now you have a majority chain with 50% of the hash. This means 2x block times until difficulty adjustment. The minority chain now has 40% working for it but 10% working against it. This also means 2x block times until difficulty adjustment. Now this is where increased block size comes into play. Having 2x block times while an inconvenience to both chains is devastating to a limited block size chain. The large block chain can easily adjust to allow for all unconfirmed transactions to make it into the next block. This means that your transaction gets through in an average of 20 minutes. The limited block size chain does not have the room for all transactions to begin with. Transactions would backlog even without the attacking 10%. With the attacking 10% account for 20% of all blocks being empty that backlog will keep increasing making the chain unusable for most people.

Now yes that is an extreme example at 60/40 fork and half hash attacking but it's a valid example. It gets even more skewed with a 75/25 split and even a 5% attack. The only option is to change to something besides sha256. That means all existing miners on the pool now have 3 options. First turn the miners off and let them be expensive bricks. Second try to attack the majority chain by mining empty blocks. The first makes no money. The second looses money and actually helps the majority chain by getting to difficulty retarget faster. Transactions can still fit into the next non empty block. That leaves the only viable option of that equipment being used to mine the majority chain. They can do it themselves or sell the equipment to those that will to try to cut their losses.

It really is this simple when it comes down to it. They either switch or go out of business. Sure they could try to mine some other sha256 coin but that would most likely be a loss in the long run. Before anyone says the majority won't attack the minority based on some kind of principal. Again it's money. It's also the way the system was designed. Before the whole ETH/ETC thing this is the way forks were described. Majority attacks and kills the minority. Why didn't it work with ETH/ETC? Neither has a majority of the hash rate. People could just fire up more GPU mining for one or the other. This is an actual benefit of ASIC mining. If a majority of all ASIC miners are already mining one coin you can't just switch from other coins to prop up one chain or the other. With CPU/GPU you can. With ASIC you have to build more or try to put old unprofitable units back online. Both cost money and the second doesn't have a chance of making any.

So this is my .02 on why when a fork happens miners will have to go with bigger blocks. With Bitcoin Unlimited already near 33% and 8 meg block signaling another 8 or 9% I think we are nearing a tipping point where miners will start to vote with their wallets instead of their political beliefs.