There's something similar going on here on the edge of Manchester. We bought new in 2017 and our estate was finished in 2019.
In our case, the new development didn't appear on our searches but the whole area has been earmarked for more homebuilding for years and it was as clear as daylight in the Council's long term housing plans. Besides, anyone who thinks that new developments in desirable areas will be immune to future building in adjacent fields exactly the same as the one they've just bought is living in a fool's paradise. I've got a couple of theories about why people are selling though.
First, I am sure that some don't want their views built on. The houses looking directly on to the new site were the very first to be built in 2016, so it's entirely possible that buyers who didn't have the full picture are packing up in righteous indignation. Or, they find the work itself disruptive with noise, dust and so on - particularly if they are now working from home.
Second, the builder at the new site has less of a reputation for quality than the (very good) ones that built our estate so our slightly older homes look particularly good in comparison at the moment. The new development brings people to the area though.
Third, I know a lot of the buyers here bought with Help to Buy. I suspect that this development was not their first choice at the time but they bought because they had little other choice with their budgets. For anyone with little savings/deposit this has enabled them to rapidly build up equity, which might now enable them to buy something they want to, rather than to a new build.
Fourth, the market round here has gone beserk full stop since the pandemic began. Some will simply be cashing in. I know of one family that are seriously thinking of selling up and buying a crappy terrace in a worse area because they could now be mortgage free.
Finally, on an estate our size (nearly 400 houses, almost all four bed detached) there is simply always going to be a churn level, it's just that this might be higher at the moment because of covid-19 and other economic disruptions.