Q: My cat seems to like to climb up onto all our high surfaces (knocking ornaments off in the process!). Why is this?
A: Cats have evolved to climb and use high perches as safe places to survey their territory. It's perfectly natural to climb and if cats are not provided with suitable places indoors they will find unsuitable ones!
I would suggest you keep your breakable ornaments in glass cabinets and consider providing a tall cat scratching post with high platforms where he can sit and watch the world.