I believe there are runs where form doesn't quite click. This happened a few days ago. I haven't run on mountain trails in about six weeks, then tried a fartlek on some fairly technical trails with steep grades. It was tough. The best way to describe it is "rusty." My foot placement wasn't great, I had trouble seeing the trail... I just hadn't done it for awhile. My technique was a little off. This may be different than the "bad run" experience, however. The runs where I just don't feel motivated, it's harder than normal, or just doesn't feel right could be attributed to a wide range of physiological, psychological, or emotional states. Stress, lack of sleep, too much sleep, spicy Mexican the night before, weather... all could play a role. The interesting question- would any of those factors (or others) affect gait?