What causes it to yellow? I've had it 10 years and never saw this happen.
In my research I found many reports of the Umbrella Pine taking on a yellow cast to the needles during winter with the tree shedding these needles and returning to it's normal green color during the growing season.
A soil test is a good idea to check the soil, as this tree prefers a slightly acidic soil.
It is possible that a yellowing tree could indicate a more serious issue and I've listed this link to give you more information and what to look for.