The way the cosmological argument from contingency is often put, it sets things up in the mind of the reader that there are logically only two possible sorts of beings
(i) contingent and dependent beings.
(ii) necessary and independent beings.
So that anything that is contingent is dependent on something else for its existence, and anything that is independent exists of logical or metaphysical necessity. But one wonders why this must be so. Why can't there be a third possible sort of being:
(iii) contingent but independent beings.
Beings of type-(iii) are like beings of type-(i), in that they exist in some worlds but not in others. But they are also like beings of type (ii), in that they depend on nothing else for their existence: they're free-standing beings (incidentally, this is how Swinburne thinks God is. If it's ok and coherent for there to be a god like this, why not for some non-divine beings?).
Thus, here's an epistemic possibility: there are just two sorts of beings: contingent dependent beings and contingent independent beings. The latter are the fundamental constituents out of which the former are composed. In this scenario, then, the contingent and dependent beings can come into being when two or more contingent and independent beings are combined, and they cease to exist when they decompose into their elements. But of course the elements of which they're composed can't pass away, for they are independent beings.
On this picture, then, we have an explanation for all contingent-and-dependent beings in terms of contingent-but independent, free-standing beings.
The Principle of Sufficient Reason (PSR) -- which plays a major role in the argument -- will have none of this. PSR entails that if there are any contingent-but-independent beings, then they must be further explained in terms of the more metaphysically extravagant, modally extreme necessary-and-independent beings. But surely this is explanatory overkill: the contingent-but-independent beings are enough to explain the existence of contingent-and-dependent beings. But if so, then the epistemic possibility of contingent-but-independent beings is an undercutting defeater for PSR.