We planted 5 hybrid populars for quick shade about 4 years ago. They are about 20 feet tall now and seem to lose their leaves way early. Like begining of August each year. Is this normal for WI and this species?
Poplars (not too popular, depending on who you ask) tend to be thirsty trees with a knack for dropping roots into the water table. A recently planted tree or a tree growing well above the water table may not have roots deep enough are is not getting sufficient water once the drier summer weather hits.
Dropping leaves and early dormancy is a fairly common way trees deal with drought stress.