Answer
Hypothesis: Above an intensity of 2000 foot candles, the number of photosystems and/or other critical parts of the photosynthetic apparatus becomes limiting.
Work Step by Step
As light intensity increases, at some point there is just not enough cellular machinery to handle the input light. The first place where this would show up would be in the absorption of light by photosystems--more photons would be arriving than could be absorbed by the existing photosystems. There could also be an issue caused by machinery downstream from the photosystem: that machinery might not be able to accept all of the excited electrons, which would cause a backup at the photosystems. Not having enough photosystems is, however, the simplest and most probably answer.
