Your crabapple (Malus) may simply be suffering transplant shock. It's not unusual for a recently transplanted tree to develop a few yellow leaves. Unless the problem progresses, you may not need to do much. Keep the tree adequately watered for the first growing season. You may need to water it several times a week until the new roots expand into the surrounding soil. Check the planting hole for moisture. If the soil feels dry an inch or so below the surface, it's time to water again.
It is also possible that your tree has apple scab -- a fungal disease common on apple and crabapple trees that is ugly and prevalent in many parts of the country. Humid, rainy, and warm spring weather promote the growth of the fungus, which begins as olive green spots on the foliage. The spots soon turn black, elongate, and develop a velvety appearance. In midsummer, the leaves turn yellow and drop, leaving an unsightly skeleton of a once-lovely tree. If your tree is susceptible to scab, it will need preventive fungicide sprays in future years. For best results, begin spraying in early spring just before the flowers bloom and the leaves unfurl. Multiple applications are necessary to keep one step ahead of the disease. See the fungicide label for instructions. The best way to avoid scab is to plant a cultivar that is resistant to the disease.