A group of developers looking to create the "killer bitcoin app" foresaw that the 1 MB block size limit would eventually cause problems that need to be resolved, the solution they devised was BlockStream. An increase in the block size makes Blockstream no longer the "killer app" they hoped it would be, and so they appose the increase. It's that simple. (and this is very dangerous for bitcoin)

Now they are in a position in which they have invested a great deal into Blockstream on the false assumption that block sizes would never be raised. The proposal by Gavin and Hearn challenges their pet (money making) solution that they have invested in.

Gavin and Hearn will not be making profits from Bitcoin XT, but you had better believe the developers of Blockstream will be profiting from the block size remaining unnecessarily low.

This "split" is being caused by the developers who are deeply invested in Blockstream, and the lack of consensus of the developers is what is triggering such uncertainty in the Bitcoin Community. These "small blockists" who have a financial interest in keeping the blocks low are causing this VERY DANGEROUS rift in Bitcoin, not Gavin and Hearn.

A fork that increases block size takes at least 6 months to accomplish, and if we wait until a massive increase in adoption occurs and people discover 8 hour transaction times and very very high fees, Bitcoin will be destroyed in the eyes of the masses. Gavin and Hearn are right to push this change well before it becomes an issue to avoid the kinds of consequences that would cause massive problems in the confidence in Bitcoin in the eyes of the masses.