The relationship between oxygen and pH is often overlooked as part of a medium. While pH is strictly the "potential of hydrogen"(concentration), oxygen plays a crucial role in regulating, indicating, and interacting with that balance. Possible leached nutrients? This process is accelerated by over-irrigation. Along with the combination of hypoxia or anoxia. At the bottom of the pot, the environment is dense, compacted, and lacks oxygen; anaerobic (no-oxygen) conditions exist, skewing the pH down. Topsoil has far more oxygen, but it may also be overfed with too much Ca, Mg, K, and Na. Ph of 8 indicates a base saturation(in medium) of cations above 100% capacity. Soil compaction is a bitch, and those fabrics are not half as airy as you think they are, The wet/dry cycle of your grow cycle must match the medium size; you're maybe moving water too slow from the pots to the atmosphere, allowing moisture to stagnate, you need to incorporate negative pressure into your grow space, if not then you need to run your growroom a little drier and a little warmer than you normally would to assist with evaporation, if soil compaction goes above a point, a root cannot penetrate, then it cannot remove water from a medium through transpiration, if the roots cant do it, and there is no evaporation occuring, then water becomes trapped, anerobic fermentation, acids etc yada yada all that good stuff. Good luck.