A hard fork can be said to occur when a significant portion of the economy decides to adopt a version of the software implementing blockchain validation rules that are incompatible with the previous rules.

In theory anyone can hard fork a chain, because they can start running a different version of the software of their own design. But this is pretty meaningless if no one else adopts it. What makes a hard fork meaningful is people deciding to adopt it, most clearly when everyone or nearly everyone does.