Worms

 Hope you knowledgeable people can help me here. At end of of August went out to do the night time slug collection with torch as per. I couldn't believe what I saw. I thought there were 100s/thousands of them on my lawn. Then the things disappeared under ground. I then realised what I was seeing were worms, coming to the top of my lawn. They covered the surface more or less, so I stopped in case I trod on them. It happened for some nights. Why? was this some mating ritual, or a full moon worm ritual? Then we found a hedgehog and frog in the garden, so stopped the nightly hunt.Would love to know what it was all about!