I bet it had to do with the lighting method. You said it was flashing, meaning it was on a capacitor, and it was most likely led lights, so I'm guessing what's happening is when the capacitor initially lets out the electricity, it's frequency is off a little, causing the led to mimic the effect.
An LED is never going to emit anything but a single frequency (okay, a really, really tight band of frequencies). It's either conducting, and emitting light at that frequency, or it's not.
I'm with CornedBee on it being some kind of illusion.