The tail-eating you mention, though obviously problematic, does have a kind of virtue if you look at it a certain way. Most of the subgenres, especially at the underground level, seem to have quite clear musical objectives. Sticking with Black metal, it tends towards trying to finds ways of expressing either a bleak depressive atmosphere, or some kind of extreme misanthropic hatred or a more cosmic occultism. As bands come along and appear to go as far down that path as it seems possible to go, others have to find new ways of pushing it even further, if only to justify their very existence. Rock doesn't generally have that kind of clearly-defined focus so is more likely to keep producing music simply for its own sake. Even its innovations really are just innovation for the sake of innovation. Whereas with certain areas of BM, it always feels like it's striving for something specific but ultimately unreachable: hence its obsession with being 'true'.. You can even extend that to production. The obsession amongst some bands with lo-fi recording always means they want to go further and further down that direction, so you start with Hellhammer then end up with stuff like the whole
Les Légions Noires thing and then you have bands that follow them, trying to go even further down that route. Like I say, it has it's problems (it's essentially a purity spiral) but it's key to why certain areas of BM seem to, almost by definition, have to keep going further and further down a particular path.
EDIT: In a way Symbols point about hive minds coalescing around a problem is a great way of describing what's going on within those areas of BM, where the 'problem' is always that which gets in the way of whatever it's striving for. It also probably helps explains why certain figures within it are so easily drawn towards really dodgy political beliefs.