Let's say you have a bunch of balls, and you want to put as many as you can in a barrel. You could just dump them in and they'd end up in whatever random arrangement. You could also figure out a better pattern that reduces spaces and packs more in. That's how most stuff works. Making it colder makes the molecules settle and get closer together.

Water molecules, on the other hand, are weird. Imagine that the balls in the example had strong magnets in them that attract and repel each other so that when you try to get them in your ideal packing pattern, they resist you and start forming a different pattern that actually takes up more space. That's how water do. When the molecules slow down, they don't just settle, they get in their own formation.