So, i have no proof or anything but just some things i noticed
Meghan is of course a very attractive woman, an actress and William was apperantly a big fan of the show suits.
Catherine was excited to meet her but shortly after that she started to be hostile towards Meghan for no clear reason.
William seemed excited too but the second Harry got serious about her he was against it?
Like he thought it was cool to spent time with Meghan and meet her and be close but his unimportant spare brother schouldnt marry the girl he finds interesting.
And dont say it waa solely because of classism, because not even the Queen opposed to the marriage.
Also notice that W&K relationship got worse and the cheating rumors started at that time.
Also the story of the dresses at the wedding and all that it seemed like Catherine was lashing out on someone she has a much deeper problem with but Meghan hadnt even given her any reason yet.