Posted: 08/04/2016 at 06:50
Hello Squirrel and welcome
Are the plants growing in garden soil or bought compost? If it's garden soil you might have some little critters in there which are moving the soil about. Or it might just be that the soil has settled into some air pockets.
Have your plants got proper leaves as well as the first seed leaves yet? If so it's time anyway to prick them out individually into little pots of so that they can grow stronger before they're ready to go outside. When you prick them out use some good quality multi-purpose compost.
Hope that helps