Norway maples are usually not long-lived. Sometimes cracks in the inner bark on the trunk can occur & you might or might not see them. The cracks will slow down flow of nutrients & parts of or the whole tree can suffer and sometimes eventually die.

Yeah but 40 years is not very long for a tree. And the timing would suggest it was affected by the drought. Maybe to the extent that the normal conditions since have not been enough to bring it back completely. Whatever happened you really need a tree expert to look at in person anyway.