Who decides to have a split or a fork. How are they given the power? Who allows someone to have the power to access critical systems so that they can make a mistake and screw up Ethereum?
Any open source software can be hard forked at any time; all that’s necessary is for someone to copy the project’s code then alter it in a way which is incompatible with the original ruleset. There’s no one and nothing to stop it, apart from the fact that in most cases it’s just not worthwhile. You could, for e.g. hard fork Bitcoin to have 100 meg blocks but who would run it instead of regular Bitcoin, which everyone else is running? Almost nobody, so it’s just not worth your time or effort.