They are a variation on something called Truchet Tiles.
Step 1: Design Your Tiles
Make the tiles from rubber stamps, clay, carved wood, or any other material you like. I leave those details to you.
Or follow the procedure presented in the video in step 3 and print the resulting design. If you know any Photoshop I think you'll enjoy the video.
Step 2: Randomly Place Your Tiles
Step 3: Don't Miss the Video - It's Kind of Cool
If you are here for the video and the Photoshop workflow, I hope you enjoy it. I think it's fairly novel and may stimulate in you other ideas along these lines.