What a great question – the light bends as it passes through the atmosphere and the atmosphere is turbulent and moving around as it is different temperatures. This causes the light to bend all over the place and look like it’s twinkling!
As the light from the star passes through the Earth’s atmosphere it gets refracted (bent) in random directions on the way down, which affects the amount of light reaching your eye. Different wavelengths of light are refracted by different amounts, so the twinkling can sometimes seems to flicker, too. I think I’ve noticed this especially with bright light from nearby planets such as Mars (which is normally a reddish colour), but I may have been kidding myself about this.
Sorry it’s taken a long time to answer this. I’ve been making a video which I wanted to show you when I found out I was in the final – can you check it out?