have all of them changed colour or are the majority still green, evergreens do lose leaves just not like deciduous shrubs

I have seen other Mahonias doing the same this year - so it may be a result of the cold. I wouldn't worry if the shrub's otherwise healthy.

agree with the others here, any evergreen shrub will loose 20% of their leaves during a season, so not all is lost.

Thanks for replies.Could it be a lack of magnesium?

you could give it a balanced feed just in case, I usually lightly feed in the spring if its just some old leaves I would't worry