WAG (wild-ass guess) No.1: a bit of excess headspace with those particular shells in your H&H along with slightly rough or sticky chamber walls. Scenario: firing pin whacks primer, pushing case a bit forward due to headspace, shell ignites, pressure causes paper to adhere a bit to rough or sticky chamber, pressure then pushes head of case back against standing breech while case walls are stuck to chamber, case separates.
WAG No.2: Poor paper and slightly over-diameter chamber.
Or - Messrs Holland don't like cold weather and are trying to get you to go indoors for a nice hot toddy...
