Why do few droplets of rain fall a short time before a downpour?

What is the reason that before a heavy shower of rain, a few droplets always seem to fall? Why doesn’t the heavy shower begin at once?

While freezing water, for example, some parts of the water such as the surface freeze the first, but in the example of rain, the condensation process seems to happen much faster. I am trying to understand how this process starts so slowly yet becomes so fast in a matter of seconds.

The reason for this might appear to be due to the "size" of the droplets and drag. However, the droplets that fall before appear approximately the same size as those in the downpour, also the time difference between the raindrops is too long.