Why are the leaves on my magnolia turning yellow?

Without more detailed info it is difficult to ascertain why your magnolia's leaves are yellowing and falling. In the first instance, check for insects - particularly scale on the back of the leaves. If there is scale, spray with Yates Nature's Way Citrus & Ornamental Spray. If not,check for other insects, access the moisture of your soil (it may be overly wet) and the amount of fertiliser you may have applied recently.