Demand for fresh cadavers in the medical schools of the 17 & 1800's created a very lucrative market to anyone who could provide them.  The money was so good that, of course, the criminal element found a gold mine within graveyards, unearthing the recently buried and selling them quietly to the schools.  When the monetary  rewards kept increasing, some made the jump to straight up murder - eliminating the middleman.  See: Burke and Hare

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