The most common causes are over/under-watering, too high a level of soluble salts in the soil (plasmolysis aka fertilizer burn), too much sun @ midday.

The 'over-watering' issue could be from a collapsing or poor soil, even if you think you're not over-watering. When you're growing in a shallow pot, even a soil that supports only a half inch of perched water is unacceptable. If your pot is 3" deep and the bottom 1/2" of the soil is saturated, that is about 17% of the soil saturated - enough to root roots, impair root function, and cause the marginal or tip necrosis you inquire about.

Al