This run is a classic example of erratic walk/run pace and zig-zag across the road, quickly switching to much faster pace and a smooth GPS line.
A quick clarification - the zig-zag'ing you see on GPS tracks from run/walks are mainly caused by the fact that with the watch worn on your wrist, the accuracy of the gps signal varies as your arm swings back and forth. The zig-zag is seen regardless of the pace of the run.
With the GPS watch stationary within a moving vehicle, or even on the wrist of a moving cyclist, the accuracy of the GPS signal is more consistent, and so the straighter line. You can even see this effect by placing your watch flat on your head under a cap.