What if a substantial amount of local solar contractors are doing door to door sales? Or other locality/proximity based sales (signs, driving a car with ads on it, and the like?)
I think one factor that's missing from the explanation is the extensive media and political coverage that solar panels got: There are probably very few people by now that don't know what a roof solar panel is or who don't have an opinion on them.
So my guess is that most of those neighbors who "suddenly" decided to also get a panel, were already interested or at least curious about getting one. (In the sense of "I should totally be getting one some time, but I have no time/now idea how to start/other things are more important/etc")
Maybe the early adopter was then what changed peoples' stance from a vague idea to a concrete plan.
I'm not convinced that that's it. It's more likely that the first person who got solar installed talked to their neighbors about it, and the neighbors were convinced. It's not like after you move to a neighborhood, you're really choosing anything after that point about your neighbors.
In 2015, two geographers noticed solar panels popping up on houses in their small US state of Connecticut. Curious, they set out to see if they could figure out what predicted who had them. Would they be in richer homes? Or in areas with higher population density?
Early adopters of solar panels tend to be people who are interested in innovative technology, who find an installer they trust, and who think having solar panels will benefit them.
But once an early adopter made their choice, the geographers found, a cluster would spring up around them. Having solar panels on a house near you, where you could see them and talk to a real live person who had them, it turned out, was the biggest predictor of whether you’d get them yourself.
Soon the Connecticut study was being replicated – in Sweden, in China, and in Germany, where they actually put a number on it. Rooftop solar installations were most influential, they found, on neighbors who lived within one kilometer (source: TED ideas).
The truth, of course, is this applies well beyond installing solar panels. Solar panels are just physical manifestations of the proximity principle.
People who prioritize their health are more likely to have friends who prioritize their health. And so on.
We become like the people we choose to be around.