Many mineral and elements are required for the proper functioning of a Cell. As in the stress condition the functioning of cell changes, which results in altering the function of cell organelles (eg. chloroplast); Allover these cascade of effects results in the abnormal chlorophyll content in the chloroplast.
Now, coming on to How, there can be many factors as - absence of proper nutrients and minerals, formation of ROS, UPR responses etc.
Because the content of the plant tissue of chlorophyll is high in the improved conditions and this in turn affects the increase in the activity of photosynthesis in the plant and the accumulation of high amounts of food, causing an increase in growth. In the absence of perfect conditions, the opposite occurs and this interpretation is accept with plants under stress.