Wordpress - What is the design pattern for WordPress Core?

Spaghetti with meatballs

The term "spaghetti with meatballs" is a pejorative term used in computer science to describe loosely constructed object-oriented programming (OOP) that remains dependent on procedural code. It may be the result of a system whose development has included a long life cycle, language constraints, micro-optimization theatre, or a lack of coherent coding standards.

It also has a bit of macaroni mixed in...

http://en.wikipedia.org/wiki/Spaghetti_code#Spaghetti_with_meatballs

WordPress uses the Event Driven Architecture due to action/hook API.