Ailing Lemon Tree

There are several possibilities here and more than one of the following problems might be causing what you are seeing.
1. How long has this plant been in the pot it’s in? One reason plants can go downhill like this is if their roots are so congested in the pot that there isn’t enough soil for nutrition and to hold onto water. If your plant has been in the same pot for three or more years, it’s time to move it up to a larger pot with fresh soil.
2. Did the plant get fertilized recently with a synthetic fertilizer? If so, and it was either fertilized when the soil was dry or at too high a concentration, it might be fertilizer burn you’re seeing. Fertilizer burn turns the tips of plants brown. Be sure to never water a thirsty plant and only use the concentration recommended. Better yet, use one of the organic fertilizers formulated for citrus trees. Citrus should get fertilized regularly year round – apply according to directions on the product you use.
3. Something dumped in pot? Someone slipping the remains of their drinks, or the plant getting hit (on leaves or soil) with household cleaning products, can also cause leaf death such as this.
4. Plant drying between waterings. Although you’ve had this plant for awhile and have been watering it regularly, sometimes things can change so that your usual routine is no longer enough. Examples of such a change are being root bound (#1) or the plant being moved suddenly into more sun.