Experts and BHG readers answer.
Water sprouts or suckers
Magnolias refer moist, acidic soil high in organic matter and they must be kept mulched. Regular watering is good, but "fairly decent" soil probably isn't good enough for your magnolia, and it is stressed. Stressed trees can send up watersprouts. There are also different types of magnolias for different regions; be sure yours is a proper cultivar for your area. Check with your local county extension service.