It doesn't take much to mess up well-practiced plans. All it takes is a hockle/loop or snag preventing a line from running free, or another boat in your path doing the unexpected, or a gust, or a slight misjudgment. What if you hadn't rowed out of their way, or that couple encountered a gust that slowed them to a stop just before they reached their mooring? Would they have enough time/space to recover before they hit something near?

No doubt that well-practiced couple had a "plan B" and had readied all their lines before coming in... but "stuff happens" to all of us, sometimes.