There are all kinds of possible explanations for this. If you look at this heatmap of vaccine uptake by age and time (you'll need to select second dose)[1], you'll see that the uptake percentage is fairly uniform across all age groups below 60 e.g. in March, but goes up fast for 55-59 in April, meaning the "10-59 + Second dose" group became older on average during that time. My best guess is that the uniform distribution early on was from healthcare workers; later on, the age-prioritized vaccination rollout changed the distribution.

There's a reverse trend that coincides with an increased uptake in younger groups: The age-specific death rate for "10-59 + Second dose" in June was 2,8 and went down to 2,4 in September.

One thing to keep in mind: When we're talking about the 10-59 age group on their second shot prior to April, that's only a population of 800k with a total of 16 deaths reported between January and April. Confidence intervals for this period are very wide and the upper confidence limit in the ONS data is quite close to what they're reporting e.g. for September.

[1]: https://coronavirus.data.gov.uk/details/vaccinations?areaTyp...