I have found the following comment among the writings of a great Kabbalistic scholar, Rabbi Chaim [Vital], the leading disciple of the Arizal [paraphrased by the editor]:

As a consequence of Moses' accepting the Mixed Multitude as converts, he became involved in the calculations pertaining to leap months, leap years, etc. One of these calculations involves determining when a Jubilee year occurs [the fiftieth year after the conclusion of seven cycles of seven years].

Not only did Moses fail to truly convert them but they also infected the Israelites proper with their lack of faith….

G‑d had not wanted to accept this Mixed Multitude as converts. Had they not been accepted, Israel would have experienced neither death nor exile, since acceptance of the Tablets would have signified everlasting life, as our sages said: "Do not read 'engraved' [in Hebrew 'charut'] but rather 'free' [in Hebrew, 'cheyrut', i.e. from death].

Moses had not consulted G‑d [regarding the acceptance of the Mixed Multitude, which had been Abraham's lifework]. In addition he had developed a personal interest in the conduct of these people, as he had hinted when he referred to them as "the people amongst whom I find myself." (Num. 11:21) He had also foretold that these people would convert when he told Pharaoh that "all these people who sit at our feet will bow down to me." (Ex. 11:8) This meant that Moses was anxious to convert these people.

Alas, not only did Moses fail to truly convert them but they also infected the Israelites proper with their lack of faith during the episode of the Golden Calf, so that G‑d told Moses: "Go and descend, for your people have become corrupt." (Ex. 32:7)

These people and their offspring by now made up the majority of the Jews in the desert. This is why Moses was forced to insert an extra year [the Jubilee year] after every 49 years. This extra year serves as a warning that Israel must not again err by accepting converts wholesale and being misled by them.

[Translation and commentary by Mr. Eliyahu Munk]