OK, here's the deal:
These aquatic plants expend a lot of energy trying to maximize CO2 availability. That's a very tough proposition under water because the gas is not as easy to come by as it is in air (even when you are adding it).
In the morning, at lights on, they have a really difficult time getting their machinery running. Imagine they are a Ferrari on the track. They need time to warm up. Just because you switch the light on it doesn't mean that plants can switch everything on and operate at peak efficiency in an instant. Only algae can do anything close that.
When you drop the water level the plant leaves are exposed to air. They dry out, of course, if they are exposed for too long, but the duration of the water change is enough time to get the leaf saturated with air.....air that contains CO2 in bucketloads. So the plants are taking a massive gulp of CO2 when they are exposed. This is yet another reason I do as large a volume change as the fish can tolerate without becoming grounded in the sediment. The lower the water level, the more mass of leaves are exposed to air.
Then, when you fill the tank and turn the lights on they have gotten a head start because the CO2 is already exactly where it needs to be. Sugar production can begin very quickly. Combined with clean water, the process which removes the dirt and silt from the surface of the leaves, they get an even better head start.
Additionally, if you are using tap water, many municipalities have water which is exposed to CO2, either added intentionally, or absorbed while the water is in the aquifer or as it passes through CO2 containing sources on it's way to the aquifer. The water delivered under pressure to the tap often retains a high CO2 content. So then when the lights go on the Ferrari has a much better chance to make a clean getaway. It's like having all the stars line up.
You can get an idea of how much CO2 your tap water has by measuring the pH as it exits the tap and then measuring it after a few hours of sitting in a cup. If the pH rises significantly then this indicates that CO2 was in the water and evaporated, lowering the acidity thereby raising the pH.
If you have a non-CO2 enriched tank, then you have to worry about such large water changes near the photoperiod, and yes, a lights off water change can help avoid BBA (or just doing small water changes like 10%), but if you are dosing Excel and/or adding gas then this is the best time.
Cheers,