In the Second Analogy, Kant continually refers to the need for an objective sequence of appearances. For example, without an objective sequence, ``no appearance would be distinguished from any other'' (B238). Or, later, if occurrences did not have necessary causes, ``we would have only a play of representations'' and, again, ``no appearance would be distinguished from any other as far as the temporal relation is concerned'' (B239). Why is this the case? It seems that Kant is suggesting that a mere subjective sequence of apprehensions is too fragmentary to yield distinct appearances (and thus objects).
From a Humean perspective, it seems that Kant's argument begins with an assumption that a necessary sequence underlies the unity of appearance. From there, he establishes that causality is precisely the kind of sequentiality needed. Of course, Hume would argue, this is begging the question: the necessary ``objective sequence'' which Kant assumes is precisely that of which he is trying to prove the existence.
Suppose we grant that an objective time-determination is needed for the possibility of experience. Kant claims that time is the necessary condition of all inner experience. In particular, we perceive our own internal states within time. We thus have a subjective order of events, based on when they reach our mind; this is Kant's subjective order of apprehension. Add to this subjective order, then, the fact that we perceive time ``backwards''. That is, a thought can concern events occurring before, but not after, that thought. Given this constraint, the order of apprehension is not entirely arbitrary---we apprehend X before we apprehend an apprehension of X, for example. This is a rule, in accordance with which the apprehension of one thing follows another. According to Kant's criteria, this allows for an objective sequence of appearances. Thus, without cause and effect, but rather with the internal ordering of memory, we can separate appearances. Causality is then not a necessary condition of objects; it, in fact, does not necessarily exist at all.