Why? Princess Charlotte said to her brother Prince George that the casket was passing by and he needed to bow.

You need to bow was her words. So the hat thing is from Clint Eastwood references of when he touched his hat brim and said something funny.

So the two put together means when you see her touch the hat brim, you need to bow or she will make sure you bow

Did I explain that well enough or do you think everything is Anime?