What causes word blocks to light up in Baba is You?

All blocks light up whenever you line them up into any valid sentence.

In the image in the question, PUSH is lit up when "FLAG IS PUSH" is lined up, and "OPEN" is lit up when "FLAG IS OPEN" is lined up. It is hard to tell that PUSH is lighting up because you are moving the blocks, and because PUSH's unlit state looks a lot like its lit state, and because OPEN's color when unlit looks a lot like PUSH's color when lit. If you crop out just the PUSH block from each image you will see that they are in fact different colors.