That's reasonably probable save for "and the corpses are starting to completely decompose".

The geology of the island of Great Britain is such that it has a steady rate of coastal erosion .. a number of villages once inland "far" from the sea have been lost to the sea.

The villages of Clare and Foulness succumbed to erosion in the 15th century, that still continues to this day: https://www.bbc.com/news/articles/cpwvj80yg40o

Old churches and their graveyards are lost, previously unkonown mass burial pits are exposed as cliffs erode away and the remains (bones, clothes, shoes, etc) are lost to the water sometimes before it's even noticed.

Possible, sure. On balance, given the large numbers all at once, the theory about old shipwreck cargo being breached and freed has somewhat more weight.