Here's why:
* Parallelism: The phrase uses similar grammatical structures and rhythmic patterns to express two related ideas:
* "Foul whisperings are abroad": The first phrase focuses on rumors and gossip.
* "Unnatural deeds do breed troubles": The second phrase focuses on the consequences of those rumors (or the actions they inspire).
* Poetic: The language is intentionally heightened and figurative, adding to the sense of drama and foreboding. The use of "foul," "unnatural," and "troubles" contributes to this effect.
While this specific phrase might not be found in any specific literary work, its structure and style are very similar to those found in works of poetry, particularly those from the medieval or Renaissance periods.