Every little change in the current way something works will always have an impact on price as well as their tie to other currencies, even though it may not be as strong.

When people see bad things they flee, when they see good things they flock. And since price is set by availability on the market, the price will move largely when these things happen.

Who initiates a hardfork?

Witnesses does that. The developers release a new version, but it's ONLY the witnesses that can enable it. I don't recall the exact number, but I think it's about 60% of the Top20 witnesses that has to upgrade for the new version to be mandatory.