As I read it, the values here are the date that cherry blossom are first witnessed in the year. There's probably a bell-shaped distribution of when blossom appears on a given tree i.e. there will be a very small number of trees that bloom really early. If you have more people looking for it, then there's a greater chance that someone will spot a really exceptionally early tree. So an alternative explanation for the trend is just an increase in the Japanese population.

(Counter argument: when the population gets high enough that essentially all trees are witnessed, further population increases would have no further effect.)