Earth worms are usually closer to the surface at night and early morning.As the weather is turning milder they are just becoming more active.
The more we tarmac and pave,the less room they have to live,they are probably reproducing as normal and will find their own way to soil.
I wouldn't think of them as a plague,they do a valuable job in aiding drainage with their tunnels and breaking down decomposing plant matter.
I'm sure your local bird population will be very grateful :)

Petunia, did you mean earthworms, or some kind of caterpillar?

Thanks for the responses. They were definitely earthworms and not caterpillars. It was curious that so many appeared in the locality on the same day. Rather similar to the emergence of flying ants who take to the skies from their burrows all on the same day. As you say, it's good food for the bird population.
Nature's wonderful!