A very good question, with an answer too complex to answer in a blog site in a few minutes. Here are some hopefully helpful facts. The year-to-year difference in Nino3.4 SST is greatest in late fall and early winter, and least in late spring. During late spring the climatological average SST peaks, meaning that it has higher average SST than the other seasons, and during late summer to early winter the "cold tongue" develops  and the average SST is lowest. So, each year, normally, there is a tendency toward El Nino in spring. During El Nino, the SST is just a bit warmer than average in spring when an event is developing, and as the climatological average declines during summer and fall, the actual SST fails to do so, so that the anomaly increases. So late fall is the time when the anomaly has the chance to be greatest. There is an upper limit on how warm the Nino3.4 SST can get (not anomaly, but actual SST) and in spring the climatological SST is closer to that limit than it is in the fall and early winter. So anomalies are more limited in spring than in late fall. When SST approaches the limit, cloudiness and rainfall increase, shutting off the sun and capping further increases, even though ENSO-related dynamics may proceed. I know this answer is not very satisfying, and you need to read some ENSO literature to learn more about the seasonality of the ENSO phenomenon.