I really couldn't pinpoint an exact cause. The heat would have been turned on between when it arrived and when it started to decline, but it would have been on for a long time before I saw it decline.
My family is kind of weird with the heat. Some of them like it ice cold. It's possible the temp dropped, but I don't think it would have dropped far enough to affect the plants. I thought it was dropping a lot, because I'd wake up really cold. However, I think that's just a matter of me laying there and not moving while I sleep, so my body is cooling slightly at my extremities. Once I got the little temp and humidity guage, I saw it's only fluctuating by 5-10 degrees throughout the day and night.
I don't remember if I changed the lighting before or after the decline, but definitely within a reasonable time frame of the lower leaves wilting, I swapped the lamp bulb from a standard house CFL (5000k or 5500k I think) to a 6500k daylight CFL. All of the plants seem to favor it. They're now leaning toward the lamp as opposed to the window that's 3' away.
Besides that, the only thing that should have changed would be the amount of daylight coming through the window. I wouldn't think it's the new light bulb, though. I understand that CP's like brighter light and that the 6500k bulb is only equivalent to a cloudy day as far as lux goes.