It's like being a beginning reader - you want to read everything; signs, labels, instructions, pamphlets and brochures, newspapers, books (all kinds), the internet..., even symbols and logos (semiotics) if nothing else is available.
It's taking a book around with you "just in case". It's being stuck in a lift and reading the weight limit notice over and over, until you almost convince yourself there is a deeper meaning behind it... It's reading to engage your mind. It's reading just for the sake of doing something with your eyes, a bit like a chain-smoker wants something to do with their hands and mouth.
An addiction? Perhaps there should be clinics; "Doctor, I read too much..."