That's what you call aliasing & moiré problem -- the result of line skipping and pixel binding . Just search for aliasing & moiré patterns .
Take the 50D sensor size in photo mode 4752 x 3168 divide by 3 gets the camera resolution in 1:1 1584x1056 for video
But In 3X crop mode where every pixel is counted (no line skipping or pixel binding or at least minimal pixel binding)
So the lesson here is do not shoot -- bricks , power lines , wires anything that has fine patterns in 1:1 mode
but it's ok to shot that in 3X Crop mode where the problem dose not happen .