The 2nd dose cohort at that point had quite a different age profile (they were rolling out doses oldest to youngest). Over 50 age bands were 90%+ double dosed but 60% or less for under 50 age bands. It’s possible that older cohorts were out-performing typical mortality because of something like lack of flu due to lockdown.

Another possibility is that the first dose is an absorbing barrier if it causes so much illness and death that victims do not get a second dose. The two-dosed cohort are the “survivors” with the vulnerable already removed from the population.