Tilia tree dying, next to a perfectly healthy one

Hello there!
This tilia tree was dropping leaves all summer, from the spring it looked little unhealthy, the leaves were littlebit pale. I did not see on it disease or any harmful animal.
Next to it a perfectly healthy tilia.
Central Europe, Hungary
There is watering system, every other day.
It is an old tree, maybe 50 years old.
What can be the cause?

Thank You for your insights.