I suspect that this is not from 1577 but rather later. The medical use of cadavers was rare then but much more common later. These skeletons may date from the late 1700s and into the 1800s when much medical research was being done. It was also a period when "bodysnatchers" and "ghouls" would steal the newly dead for doctors desperate for knowledge. I wonder how long the mentioned Prison existed. These could be inmates executed much later than Jenks curse.
I'm with you. This was the courthouse/prison burial ground and the bodies are from over a long period of years. Pure speculation with regard to how the earlier subjects died.
As for the dissections, many were done illictly by the medical students on bodies provided by the "ghouls", and what better place to bury the evidence than the prison graveyard?
Burke & Hare alert!
According to this site, the county gaol has been on this site basically forever, although it was not one of H.M. Prisons until 1888. The prison was in use believe it or not until 1996. Most of the prison buildings are from the Victorian era, but it occupies most of the bailey of a castle that has been there since 1087 (and the tower keep is still there for that matter.) If they dig deep enough, they'll find 11th century burials in the old castle bailey as well.
Americans sometimes find it hard to comprehend that you can't put a shovel in the ground most places in England without disturbing something really, really old. It is fascinating to read the historical impact statements that are published by the various county councils. The London ones make especially good reading - you can almost see the poor archaeologist tearing his hair as he tries to list the Victorian, Georgian, medieval, Roman, and prehistoric remains before the bulldozers roll in . . .