why do the leaves on my laurel bush turn brown and drop off

How many leaves? If you're just talking about a few here and there, with most of the bush still green, healthy and growing, this is just normal leaf loss which goes on throughout the year with this plant.

the bush itself looks healthy but there are a lot of leaves dropping all the time, thanks for your reply

If the plant is healthy, growing, and still very bushy, then its just normal leaf loss.