I suppose one could argue that there is some change in reporting rates, since rape is well-know to be an under-reported crime. However, I would struggle to argue that under-reporting rates are going up (which is what it would take to be the prime driver of the trend above). If anything, my guess is that reporting rates are rising such that the chart above actually understates the improvement.
PS- Folks commenting on this post saying that by reporting a declining trend I demonstrate that I don't care about rape or don't treat it seriously are idiots. I have lived through dozens of data-free media scares and witch hunts -- global cooling, global warming, the great pre-school sexual abuse witch hunt, about 20 different narcotics related scares (vodka tampons, anyone?). Data is useful. In this case, knowing there is improvement means we can look for what is driving the improvement and do more of it (though Kevin Drum would likely attribute it to unleaded gasoline).
"Trend that is not a trend" is an occasional feature on this blog. I could probably write three stories a day on this topic if I wished. The media is filled with stories of supposed trends based on single data points or anecdotes rather than, you know, actual trend data. More stories of this type are here. It is not unusual to find that the trend data often support a trend in the opposite direction as claimed by media articles. I have a related category I have started of trends extrapolated from single data points.