The key here is to recognize the distinction between a community as a whole and the individual species within that community. Just because individuals have the capability to self-perpetuate, doesn't mean that a community as a whole does.
For example, imagine an island with a whole bunch of caterpillars. Now imagine that you introduce a non-native caterpillar-infesting wasp. On the mainland, the caterpillars have evolved resistance to this wasp and so can fight off the infestation. But on the island, they never evolved that resistance, and rapidly succumb. Upon introduction of the wasp, you have a "community" of wasps and caterpillars. The caterpillars can reproduce, as they have been, so they're self-perpetuating within their species. The wasps can also reproduce by laying their eggs in the caterpillars, so their species is self-perpetuating. But the wasp infestation is killing caterpillars faster than they can reproduce, so the community is unstable. Over time, the population of both wasps and caterpillars will crash as all the caterpillars die off. Each species can self-perpetuate, but the wasp/caterpillar community, taken as a whole, does not.
That, in a slightly more complex way, is what happens with non-climax communities. The pioneer species change the environment in a way that permits other species to grab a toe-hold. These species are more suited to the changed environment, and thus push the pioneer species out.
For example, species which can survive in nutrient-poor soil grow and die in that soil, enriching it with nutrients. Now that the soil is no longer nutrient poor, faster-growing plants can grow in the soil, and push out those species which started there. Each individual plant can reproduce, but the community as a whole is changing from one which favors the nutrient-poor adapted plants to the nutrient-rich adapted ones.
It's only in a climax community where the members of the climax community create/maintain an environment which is most conducive to the climax community species. This is why it (the climax community) is being called "self-perpetuating". The community as a whole is maintaining an environment which favors those species which make up that community.
Wipe out the climax community species (e.g. through a forest fire), and you no longer have an environment which favors the climax community species. A new set of species comes in, changing the environment, which then allows other species to come in, changing the environment further, which then ...
