While you are right about pH. Time and temperature are often a cause. Too hot a sparge is a common culprit.
"Steeping too long, too hot, or too wet. If you steep specialty grains, it’s possible to extract tannins with too much time, temperature, or water. Simply limit the steep to half an hour or so, make sure the temperature doesn’t exceed 168-170°F (76°C), and use 2 quarts of water or less for every pound of grain (4 liters per kilogram)."