Conventional theories include hail and fireworks that confused the birds.
Here's my theory and first speculation as to what could have happened (no clue if this will hold up since I didn't really research it):
Just prior to the recent Haiti earthquake there was an increased level of ultra low frequency (ULF) emissions detected by satellite. These emissions are thought to be a result of piezoelectric properties of the earth's moving crust (right?).
Some animals are thought to be sensitive to ULF emissions, and perhaps even able to "predict" earthquakes in a sense (not really proven).
What if, the birds and fish were affected by ultra-low frequencies or seismic activity in Arkansas? Or--What if the deaths are resulting from geothermal conditions that are changing in the area, causing both changes in water temperatures, pH, and seismic activity? Why not? Earlier this week there was an earthquake in Indiana in an area where there is not "supposed to be". Maybe Arkansas is a new Caldera or the next Yellowstone?
Well, I've had my fun with rampant, unfounded speculation for now. Only time will tell...
Goodnight.