Is their taboo a remnant of some pre Islamic belief?
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Prohibitions against pork go back pretty far, at least to ancient Syria.
The Torah (OT of the Bible) forbids eating pork in Leviticus (11:7-8) and Deuteronomy (14:8), and of course this predates Islam by at least 1400 years. This ban is still in effect in many (most?) Jewish sects and households.
Islam is heavily based on the Judeo-Christian "Prophets" and scripture that came before it.
I'm not exactly sure if there is a known basis in the ban, but pork does have the capacity for harboring harmful diseases that can be passed to humans through consuming the meat, if not properly slaughtered and prepared. This may have been observed in pre-history.
There are some other theories on the ban as well.