Usually from too much water. Sometimes it is from overwatering and sometimes it is from poor drainage in your planting medium. If the soil gets like sludge when you water it, you may want to use a lighter mix when you pot it up.

Over-watering, under-watering, soil compaction, root congestion, a high level of dissolved solids (salts) in the soil solution, toxicities - especially of fluoride, can all work individually or in concert to cause necrotic leaf tips and/or margins in almost all terrestrial plants. The best way to avoid these issues is by way of using a medium that holds little or no perched water. Perched water is water that occupies spaces between soil particles, robbing the plant of the O2 it needs to fuel root functions/metabolic processes. This type of medium also allows you to water to beyond the point of complete soil saturation, so you're flushing accumulating salts from the soil when you water, without the grower having to worry the soil will remain saturated so long it limits root function or wrecks root health. Ideally, soil water would be found as a microscopically thin layer on the surface of soil particles, inside of porous soil particles, and at the interface where where soil particles contact each other.

Other ways to combat the issue include potting up before the root/soil mass can be lifted from the pot intact, watering with any form of deionized water. That could be water from an r/o filtration system, rain water, snow melt, condensate from air conditioners or dehumidifiers. I have a small r/o water filtration system (makes 75 gallons per day) I use for all of our drinking water and for watering plants I keep indoors over winter.

Aside: Our job as growers is to identify what's limiting our plants and correct the limiting factor to the best of our ability, very little or nothing more than that. When we ask plants to tolerate cultural conditions at or near the limits they are genetically programmed to tolerate, it causes stress that manifests in symptoms like growth rate, vitality, appearance, yields (food plants), and the plant's ability to defend itself.

Al