Render has an obvious grid pattern

In the attached image, there appears to be a grid pattern that continues along the whole thing. I have put some of the grid with red dotted lines, which continue further, parallel.

Why does it happen?

This is a saved image, right, JPG or something similar? These are most probably compression artifacts. Use a lossless compression format (png, exr, tif etc) or render image with more samples to reduce noise and thus visible compression artifacts.