The path from strong El Nino to a La Nina of some given strength often does not occur in a straight line, but rather with some bumps in the road caused by disruptions in the formation of good ocean-atmosphere coupling, by intraseasonal activity like the Madden Julian Oscillation, or others. So it is not unusual, particuarly if we look at 1-month data. Regarding the effect of climate change, it so far has not had a great effect on ENSO behavior. There are some minor ways it has affected it, such as the greater warming in the western tropical Pacific than the eastern. But as far as causing flip-flops during a period of transition, I don't know of evidence along those lines.