Vortex shedding alternates as each new vortex forms and forces its predecessor downstream

Evidence

The shedding process is interesting to watch (figure 14.3). As the wind speed is increased, the two Föppl vortices begin to oscillate slightly, and then one of the them oscillates too far out into the flow going by the cylinder; it gets pinched off and swept away. After being torn away, it drifts downstream. A nascent vortex remains and grows in its place behind the cylinder, helping moments later to push the second, older vortex out into the flow, which is then pinched off and torn away in its turn. The whole process repeats itself thereafter with the vortices alternately reforming, wagging too far out, and taking turns being torn away. — Eric J. Heller, Why You Hear What You Hear- An…, 1282