Here's why it might be used:
* "Bonanza" itself is an English word derived from Spanish, meaning a period of great prosperity or success, often associated with gold rushes or other sudden windfalls.
* "Spanish" could be used to add a sense of exoticism or a connection to a place known for its history of wealth and exploration.
However, without context, it's hard to say exactly what someone means when they use "Spanish bonanza." It's likely a colorful and slightly informal way to describe a positive situation.
For a clearer understanding, you would need more information about the situation in which the phrase is used.