This book is a logical study of ‘sudden’ and ‘suddenly’ in the Old and the New Testament by a renowned scholar on the subject. Discussion includes the events surrounding the usage of this vocabulary as well as the import behind the variations involved.
In English, the word "sudden" started out by having an association with terror and gradually became neutral. Similarly "pith'om" and "petha" in Hebrew underwent a similar transition. The root "bt" indicating the seizure of a person by some terror is said to be identical, originally, with "petha". (p2f)
When the Lord spoke suddenly to Moses, Miriam and Aaron, it has threatening overtones because of the rebellion of Moses' siblings. (p3)