One of the mistakes that new homeowners make is planting trees without regard to their ultimate size, especially when many can be bought cheaply from big box store type places. This certainly happened in the 50's and 60's in the suburban areas. At least two of those trees are fast growers. Your mention of moss often means an area of too much shade (for some purposes) and perhaps overcompetition from roots preventing other plants from growing. You may have a case of trees shading each other's lower branches. You could either limb up some trees to let in light and air and get a long handled pruning saw to do this and to cut away dead branches. See what happens. You may want to remove some trees too if they seem overcrowded for your lot size. Make changes, observe results and go from there.

Overcrowding could certainly be a main or the main cause. We are watching it grow to decide what trees to remove. The reason I wondered if there may be another cause is because even the most dominant trees (the oak in particular) are not doing especially well and I've seen them growing more densely in forests without the same level of decline. Thanks for the input!

There has been an issue oak decline in the Mid Atlantic, especially red oaks from what I recall. You can also take samples of leafy branches to your local extension office if you see anything unusual while you are looking things over. They may be able to identify any issues and suggest what to do.