Historically, a large percentage of tops have occurred in the April/May period. The Super Bowl indicator works purely by coincidence, but I believe there is logic as to why so many tops are clustered around the April/May period.
The period leading to the end of April is the heart of tax refund season, where Americans are flush with cash. Retailers tend to see increased spending during tax refund season and some of those refunds find their way into the market as well. On the margin that leads to a higher market and in the stock market higher prices lead to yet higher prices as the herd grows bullish. Not only that but April is the tail end of a six month period of strong inflows that begins heading into Christmas.
Once May hits suddenly there is less cash flow heading into the market. At the same time sentiment is likely extreme because the market has been rising for so long. That explains why so many tops are clustered around this period.