A) Over-watering and the root damage or dysfunction that accompanies it.

B) A high level of dissolved solids in the soil solution caused by high fertility or an accumulation of fertilizer salts and other various salts from tapwater that accumulate when you don't or can't water so you're flushing the soil when you water.

C) A nutrient ratio in the soil solution that varies significantly from the ratio at which plants actually use the nutrients. This is caused by fertilizers with inappropriate NPK and other nutrient ratios, and by improper watering.

D) An accumulation in plant tissues of any element(s) known to cause symptoms of toxicity in given plants (like fluoride), even at relatively low levels.

Those would be the main suspects, though there are several others related to cultural conditions like temperature, light levels, degree of root congestion, soil compaction, ........ that could also cause or exacerbate the symptoms.

Al