Do you have nerite snails in the tank? The pictures aren't really clear but Pond snails eggs laid in a tank don't look like the pictures you posted.
Below are a sample of first pond snail egg cases and the second is of nerite snail eggs. It is uncommon but not to say it doesn't happen, nerite snails really only lay eggs in Brackish water not fresh. However they could have hitchhiked in on a decoration or plant. But you would have seen the actual snails before this in the tank.