The same thing happened in my tank. I added new nassarius snails and the one that I had before started to twist and fight or mate with one of the new ones. After about 10 minutes they just sat there together for about an hour then went on their way. I have yet to see any baby snails and this was months ago. I have also not seen this behavior again.
By the way you picked a great snail. They will eat dead things in your tank very quickly lots faster then a crab. I have seen 4 nassarius snails eat a fallen astrea snail in less then 5 minutes. That would take 4 crabs a half day.