I opened the image in an editor and replaced the yellow lines with red ones, which made the lines look a lot straighter. I think the illusion comes from the color of the yellow interacting with lines suggested by the white background “pebbles”.

There’s not; your eyes see the vague white lines formed by the lighter boxes, but when they’re in your peripheral vision your brain is filling in the missing information, and because they’re so close in location, colour, and thickness to the straight lines, your brain conflates the two. This results in exaggerating the white lines so you’re more aware of them than when you’re looking straight at them and can see the outlines of the white boxes clearly, and the white and green lines seem to blend when they’re both in your peripheral vision.

I find going out of focus makes the grid appear straight

Look at the white zigzags, they’re what trick your brain. If you switch to the square lines after keeping the white zigzags in mind you should be able to see both and your brain doesn’t switch back and forth between them.

Close one eye and tilt your phone away from you slightly