Here are some possible reasons why you might be adding that extra "s":
* Pluralization: You might be thinking about the word "union" as a plural noun, which would be "unions".
* Typo: It's possible you accidentally typed an extra "s" while writing.
* Regional dialect: In some dialects, words might be pronounced with an extra "s" sound at the end, even if that's not the standard spelling.
The correct spelling is "unions".