Here's why:
* Imagery is the use of vivid and descriptive language to create a picture in the reader's mind. "Frothing" and "fragrant" appeal to our senses of sight and smell, painting a picture of something bubbly, perhaps a drink or a cleaning agent, and something that has a strong, pleasant aroma.
While "frothing and fragrant" might be an example of sensory details as well, sensory details are simply the use of words that evoke the five senses. Imagery goes a step further by creating a more complete picture in the reader's mind.
Let me know if you'd like to explore other examples of literary devices!