Posted: Today at 08:00
There's a possibility that the early dry weather in the year is having an effect, similar to Acers dropping leaves early. It's the lack of water in spring rather than anything happening now.
We already have a lot of dropped leaves from trees up here, so it also depends where you are in the country. Remember that they're willows, so, when they're in a pot, any spell of being short of water has more effect than when in the ground.
In any case, Verdun's advice is good so follow that