My baby used to laugh quite randomly and I rarely knew why he was laughing! He was quite a solemn chap most of the time and usual things that make babies giggle, like peekaboo and tickling, would leave him looking slightly puzzled and definitely not laughing....
When something did make him laugh, he had a chuckle like a dirty old man that was so incongruous that it always made me laugh too! Rather disconcerting for anyone else near by though....