Love the wiki for quickfire and reasonably adequate explanations!
hardfork occur when some people supporting that coin is not satisfy to the current protocol.
My added understanding, possibly not incredibly accurate.
That is one of the scenarios for a hardfork, but not the only one. It is possible for a hard fork to occur with consensus, i.e. all current users (nodes in this case) support the fork and everyone upgrades software following new rules, as I understand, usually because of a critical security risk.
A technical hard fork could also happen inadvertently via blockchain reorganisation (client discovers longer chain and abandons shorter one). Unlikely to ever happen again at current network growth but I read that this happened at least once already even with Bitcoin on 12 March 2013... and I see can be common with alts.