猛然 emphasizes the forcefulness and abruptness of an action, not just its suddenness. Compare with 突然 (tūrán, suddenly) which is more neutral, and 忽然 (hūrán, all at once) which is softer. 猛然 implies physical or emotional intensity.
grammar
猛然 is placed before the verb: 猛然 + verb. It is more literary than 突然 and appears frequently in narrative writing.
