What's your question? Ask

Do you know the age of the Poplar Trees?
The Poplar Tree is not a long living tree. 50 years for a healthy tree, but environmental issues, insects or disease can shorten the lifespan.

The chances are all the trees in you area were planted at the same time, so if they are all showing the same signs it most certainly could be age.
People plant Poplars because they are fast growing, but it does not have the long lifespan as I mentioned.
Of coarse inspect the trees for signs of insect or disease.
Also, if you see multiple trees with issues it could be worth having a professional arborist look at the trees.
You could also check with your local county extension office to see if any other homeowners have reported tree death issues.

https://www.gardeningknowhow.com/ornamental/trees/poplar/growing-poplar-trees.htm

This article will help you:
https://www.gardeningknowhow.com/ornamental/trees/tgen/tree-sucker-removal-and-tree-sucker-control.htm

Cut the suckers and paint the fresh cuts with undiluted Round-up. This will get into the remaining root system and kill the roots. It may take a month or so, but it will work.

These articles have more information.

https://www.gardeningknowhow.com/ornamental/trees/tgen/tree-stump-removal.htm
https://www.gardeningknowhow.com/ornamental/trees/tgen/how-to-kill-a-tree-killing-trees-in-your-garden.htm

Here is an article about Poplar Trees.

https://www.gardeningknowhow.com/ornamental/trees/poplar/growing-poplar-trees.htm

Poplars should only lose their leaves in fall. If a poplar loses its leaves in the spring or summer, it is most likely sick or is experiencing difficult environmental conditions such as a drought.