Hi mate, Its just from them touching the soil. They get wet and tend to die off. Nothing to worry about and is completely normal.
But on the other hand they are looking very hungry and lower leaves dying off is a big sign of hunger. Especially when the rest are coloured the way hers are, SoI'd up the feed as well. If your in coco make sure your water is between 5.8- 6 PH
Hope this helps 🙂