Matilda Brown, PhD, University of Tasmania/ The Conversation

The short answer is that leaves fall off trees when they aren’t doing their job any more.
A leaf’s job is to turn sunlight into food for the tree. To do this, the leaf needs water. This water comes from the soil, and is sucked up through pipes in the trunk and branches all the way to the leaves – this can be a very long way for tall trees!If there isn’t enough water, the leaf can be damaged and stop working. The tree doesn’t want to waste all the good things in the leaf, so it takes the nutrients from the leaf back into the stems and roots. This way, they can be recycled.
When the leaf is empty, the tree stops holding onto it and it falls to the ground, or blows away in a gust of wind.
What are deciduous trees?
Some trees lose their leaves every year. These trees are called deciduous trees, and they lose their leaves in response to the seasons. Deciduous trees mostly come from places where winter gets cold and snowy.
When it is very cold, the water in the tree can freeze – the leaves stop working and can even be damaged by the ice crystals. These trees know to prepare for this, and start taking nutrients out of the leaves when the days get shorter in autumn – this is when we can see them changing colour.
But there are deciduous trees in tropical places where it never gets cold. Winter in these places is very dry. When the rainy season ends, the tree knows that it will not have very much water for a few months, so it lets go of its leaves.

Trees hibernate too
When the tree is leafless, it can’t make food. But it doesn’t get hungry. Instead, it rests.
Just like a bear goes into hibernation and snoozes all through winter, trees have a long sleep until the water in the pipes starts moving again. This can be in spring, or when it starts to rain again. Then, they wake up and put out new leaves, so they can start making food again.
Some trees hold onto their leaves all year long. These trees are called evergreens, because they stay “ever green”. But the leaves on these trees all die and fall off eventually. That happens when the leaves are old or damaged. Leaves don’t work very well after they’ve been munched on by an animal.
Leaves are really important for the tree, but sometimes it’s better for the tree to let them go. They can save all the good bits and when there is enough water, they can use them to grow brand new leaves.