It could just be normal closing that they will do from time to time or something serious --- really hard to tell from that picture.
How long have you had it?
How long has your tank been set up?
What lights do you have?
What size tank?
What are your current water parameters --- with numbers please, "good" "fine" etc really doesn't tell much.
How deep is your sand bed?
Have you fed it recently, if so, what?