I am trying to remember which author I read talking about this - I have a feeling it was Ben Bova - but there is a simple rule which a lot of writers of adventure stories, space operas, thrillers, mysteries, etc., follow (often perhaps unconsciously): for every problem that the hero(es) solves, you need to pose him (them) two more. This is the source of narrative drive in a page-turning book or edge-of-the-seat film. The plot takes on a life of its own as a result - and the best and most satisfying of plots is one in which the problems keep proliferating in this fashion until the resounding climax in which they all get solved at once.
Think about The Empire Strikes Back. Our heroes escape the Imperial fleet and make it to Bespin. Okay, one problem solved. But it turns out Lando is in cahoots with the Empire and Darth Vader is already there, so Han gets encased in carbonite and Luke gets chopped to bits. One difficulty gets replaced by two and the audience remains hooked. This also works on the micro level: at one point Lando manages to release Chewie and Leia. But they're too late to stop Han being shipped off... oh, and there are also loads of stormtroopers now shooting at them. And so on.
This isn't a hard and fast rule, of course, but it clearly exists as a sort of rule of thumb. What I've noticed lately is that it also seems to happen in a lot of the games I run; there seems to be a similar process at play by which problems tend to proliferate. The PCs may achieve something, but in doing so they bring other problems into existence at an exponential rate. I don't believe that I cause this consciously. It seems to be a happy accident. But it gives my campaigns (to me, the DM, at least) a kind of page-turning quality. The 'plot' (which is not pre-ordained, of course) rattles along and I'm hooked.
For example, in one of the games I am running, the PCs solved the disappearance of a group of villagers - but as a result of this they now have a vengeful demigoddess to deal with and a magic potion to track down, not to mention having to act as a go-between for two power centres and becoming entangled in an apparently unrelated issue to do with the enchantment of a young noblewoman. There is an apparently preliminary resolution which itself has ripple effects and repercussions which themselves have to be solved - and so on, ad infinitum,
This is another of those examples wherein an unplanned sandbox game causes story to emerge in a random (but very satisfying) way. But I suppose if you wanted to run a more planned, narrative type of campaign you could put this rule into effect in a more conscious fashion.