Agreed. Nitrogen Deficiency Always starts at the bottoms of plants. It's a mobile nutrient so the plant will rob the bottom leaves of it to transfer the nitrogen to it's growth tops. Nitrogen is what makes your plants look nice and green. Excessive nitrogen will make your leaves dark green, whereas lack of nitrogen will cause bottom leaf yellowing. The leaf yellows from the tip back, and yellows uniformly. You won't have green veins or yellowing between the veins. The entire leaf will yellow evenly from the tip, moving back towards the leaf stem. Then it will curl and crisp up starting at the leaf tip.
To determine why it's happening, you have to ask yourself if you're feeding enough to your plants, and is your pH in range? If it was a lock out situation you would have more issues displayed than what you do. Check Nutrient regiment and mixture, and check pH would be my advice.