We discuss measurements of the long-term creep rate from the San Juan Bautista section of the San Andreas fault. These observations of aseismic slip span a period of approximately half a century making it the longest creep record ever measured along an active fault zone. The records show systematic changes in creep rate over time scales of tens of years, and we address potential causes of these changes. In principle, they may be caused either by changes in resistive forces within the fault zone or by variations in plate-tectonic driving forces. Such driving force variations have recently been suggested to have occurred in the same area (Western North America) and within the same time window. These should give rise to observable effects in the creep data, but we find no evidence for temporal changes in plate-tectonic forcing at these time scales. A natural explanation for the observed long-term changes is elastic reloading of adjacent locked portions of the San Andreas fault after large earthquakes and possibly aseismic strain release that occurs before the earthquakes.