When I started trading many tops would happen on the day of a big news release like Intel earnings or a big employment report. Generally, we would see great news with a big gap up and a move lower throughout the day. In the past few years it has become rare to see that type of top.
Tops still occur when the news is good and the crowd is wildly bullish, but they don't tend to occur on events anymore. Most tops seem to occur at random times or right after options expiration. I don't know if this is because traders have become more sophisticated or if it is because there are more computers trading so an event does not have the same effect.