My dog could move 15 kilo rocks when he was nowhere near that weight. He's not a bully breed either or any breed noted for prodigious strength. I've seen cats move rocks heavier than they are too, and I don't doubt raccoons would do the same.
It's not so much about the weight of the rock as about how secure it is in its setting. Is the area it sits in indented, and the rock flattish on the bottom, or is it rounded and sitting on flat ground? I'd reckon putting mesh over them and tent pegs or pickets to hold that down might be an idea.
Best wishes.