Fewer fastballs but faster. Why?

I was fiddling with fangraphs.com.  I noticed that it does not provide definitions if you let the cursor hover over an abreviation as is done at baseball-reference.com.  So I'm guessing about terms.

I noticed that the percent of pitches that are fastballs from 2002 through 2011 has steadilly declined from 64.4% to 57.8% even though the velocity of fast balls has increased from 89.9 miles per hour (mph) to 91.5.

So far in 2012: 57.1% fastballs at 91.3 mph.

Why would pitchers throw fewer fastballs, especially when they could throw them faster?

Sliders have increased, curves declined:
2002 12.1% 81.9 mph    11.2% 76.4 mph
2011 14.6% 83.4 mph     9.4% 76.7 mph (curves had dropped to 8.3% in 2008)

To see data and graph, click link.