Why were Kerr's Pink spuds sprouting again?

I planted Kerr's Pinks for the first time as main crop potatoes as well as Majestic and King Edward. I dug them up today and am very pleased with the yield - nice sized spuds and only a little damage from slugs and wireworms  (I left my main crop until mid-August last year and had to throw away two sacks full with wireworm damage).
When I dug up the Kerr's Pinks I was very surprised to see that some of them had started to sprout / chit again! Only the KP's not the KEds or Majestic. The spuds were fine and I just pulled off the sprouts / chits some of them were two inches long. I'm Irish and my father grew fields of KP's and I have never seen this before.
Has anyone else experienced this and what causes it?