A careful reading of the Torah narrative would quickly persuade us that not all of the ten plagues are unleashed by Moses. 

The midrash, in fact, attributes only three to Moses--those of hail, locusts, and darkness.The first three plagues--those of blood, frogs, and vermin--are attributed to Aaron, while still three others--insects, pestilence, and the death of the first born--come directly from God. Finally, one plague--that of boils--is triggered by all three of them jointly.

The midrash does not provide an overall explanation for the pattern. From a modern perspective, I would observe that the multiple authorship of the plagues may be intended to understate the magical prowess of Moses. In its portrait of the man, the Torah consistently brings out his failings. Though larger than life, Moses never looms before us as superhuman. Moreover, at the end, the Torah conceals his place of burial. Moses is to bevenerated, not worshipped. His persona ought not to become a cultic focal point.

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