What is causing this to happen?

[morganza]

Do you have another card you can try? If it happens with more than one card, I'd say you could rule out a memory card issue, as it seems *highly* improbable that two cards would develop the exact same unusual problem at the same time.

If it happens with multiple cards AND multiple lenses, then it's either the camera or the computer you're using. Have you got access to another computer you could use to view them on?

Thank you!