Hi Nelly and welcome to GoY. When did you plant the laburnum and have you kept it well watered?

Leaves tend to go yellow when the roots get very dry, so the plant should be OK if you give it a good soaking for the next few days.

welcome to Goy from me too
I also think it may be due to lack of water and the weather is warm and certainly breezy here so that wont help either.

Give it lots of water daily and keep this up for several weeks unless we get lots and lots of rain.