Q.weeping pussy willow
Bought a plant in the spring potted it in a large container and it’s been fine up until recently when some of the leaves have started to turn brown and fall off. Can you tell me what is the cause of this, please?

A Weeping Willow, or any Willow is not well suited for a container planting.
They are fast growing and outgrow a container quickly.
You may be able to grow it for a short period in a container that is at least 15 gallon sized. A half whiskey barrel may work.
Watering certainly could be the cause of browning and dying leaves.
The tree could be too dry or too wet.
Proper drainage is very important.
Here is a link to refresh you on the care requirements.
https://www.gardeningknowhow.com/ornamental/shrubs/pussy-willow/growing-pussy-willows.htm