Lemons are often grafted to Flying Dragon Citrus trifoliata, which is selected for its vigour, pest and disease resistance, and heat and drought tolerance. They were largely regarded as rubbish thorny citrus, filled will huge seeds and serving no real purpose until the late 1800's when it was discovered they made perfect hosts. What the less seeded and juicier varieties lacked in resilience it made up for, and now you'll find just about every citrus grafted to rootstock.
Make sure you cut the shoots off as soon as you see them, close to the trunk, with sharp scissors or secateurs. If you snap them off, it can encourage more to grow. Generally, after the first few years, they will stop shooting.