Because of the light? Or because they get scared? My understanding is that the lightning releases nitrogen, which is a very good thing for any outdoor garden.

"The intense heat produced by the lightning forces the nitrogen to bond with oxygen. This makes something called nitrogen oxides. These nitrogen oxides are produced and mixed in the air and when these compounds eventually settle out of the sky or fall out with the rain--they bring a now usable form of nitrogen to the soil.

Is there really enough lightning out there to make a difference? Well, every day lightning strikes the earth over 8 million times somewhere around the surface of the planet. But remember, the lightning doesn't have to strike your garden to deliver the nitrogen. The lightning just helps the nitrogen in the air combine with rain, which then falls gently to earth."----from the US Gov.