You shall not wear cloth combining wool and linen. (Deuteronomy 22:11, JPS)Maimonides, one of the renowned Jewish scholars of the Middle Ages, claimed to have determined the reason. On the view that some of the obscure rules were intended to teach monotheism and to distance the Jews from surrounding nations' idol-worship, Maimonides had reviewed some of the books available to him that gave details of pagan rituals. He writes:
For the same reason [that it was the custom of idolatrous priests], the wearing of garments made of linen and wool is prohibited; the heathen priests adorned themselves with garments containing vegetable and animal material, whilst they held in their hand a seal made of a mineral. This you find written in their books. (Guide for the Perplexed, Part III, Chapter 37)Apparently, back in the day, wearing animal/vegetable mixed clothes was to dress like a pagan priest who was prepared 2 parts out of 3 for an idol-worship ritual. At that point he did not mention exactly which books he was citing, though based on books he mentions elsewhere, the first place I might check is the ancient work Nabatean Agriculture, should an English translation ever become available, or an on-line copy available via web translation software.