This chart is just a snapshot in time. It's hard to infer any sort of 'change' from it. Perhaps the newer members of HN have always been more active contributors (I can think of several reasonable ideas as to why that might be).
Indeed. I also thought: "wasn't it always like this?". I understand newcomers tend to be naturally more active. But I also agree that the more subscribers HN has, more is the probability of its quality going low... Maybe a graph with the average of downvotes per articles over time could help.
It tells that newer users seem to be more engaged, but it doesn't tell if we are seeing a trend or not.
Maybe as users get older and get over the reddit factor or get busier they post less often ?