Pat's right on the money. To create the chemical reaction that releases light from the phosphors in the backlight tubes, a certain specific number of free electrons are needed which come from the cathode in the tube.
When the current is first applied to the cathode, it takes some time to get up to that level. When it does, it often overshoots the mark and then quickly stabilizes at a lower level. This appears as a brief initial flash of light that quickly dims.
At that point there are enough free electrons in the tube to sustain the reaction that generates light, but in the first few seconds and even minutes of the reaction, there are still phosphors that can accept an electron that are not being filled. You might say that there is a glut of electrons. Eventually, the supply and demand sides of the curve meet and you get the final result.
The same thing happens with CRTs, that's why graphics designers are supposed to turn their monitors on at least fifteen minutes before they start working, for color-matching purposes.