My street has a row of maple trees, about 30, one on each front lawn planted at the same time 14 yrs ago. Every year half the trees from one end of the street to my house leaf out well before the other half of the street and conversely in fall the other half turn color and drop before the first half. This year my tree, along with half the street of maples, dropped a larger than normal amount of seeds and have a very noticeable thin leaf coverage with smaller leaves. No signs of disease, dead branches, leaf spot, etc. The other maple trees past my house to the end are as they always have been - dense thick, full size leaves. My tree you can easily see through to the street with shiny thin leaf coverage. Any thoughts, concerns as to why?
Maple trees produce more seeds when they are under stress and the thinning canopy also supports this. The symptoms that you describe could either be a result of an ongoing pest attack or a reoccurring fungal disease. It would be hard to tell which it is without examining the trees. But, you can treat the trees with both a pesticide and a fungicide and that will clear up most issues that could be causing it. I would recommend neem oil as it is both a pesticide and a fungicide and is systemic, so you would not have to spray the whole tree in order for it to be effective. You just need to spray as far up as you can reach and the tree will absorb it and spread it to the rest if the tree.