If your trees are growing in a woodland situation then they could well be self-pruning, dropping branches that aren't getting as much light as the ones higher in the canopy.

And some trees are naturally clean-stemmed for a long way up (Agathis australis comes to mind). As they move from juvenile foliage/structure to mature then branches are shed and the adult form begins to take shape.

However, it would certainly pay to check the trees for insect pests, too.

If you've had drought conditions any time in the last five years, that can cause slow die-back and branch shedding. And, if you live in an area with strong wind gusts more than steady prevailing winds then they can delimb even old, established trees.