This is probably just the expected effect of disambiguation. The author-date references in the main text point the reader to an entry in the bibliography. Where the author and date would point at more than one entry, the processor adds names in an attempt to identify the entries uniquely. Some styles add a letter after the date (as in "Smith 1990a", "Smith 1990b"), some add the authors' given name initials, some add the authors' full names, and many do a combination of these things.

To be sure that's what is happening, check whether, for the affected citations, the first author appears more than once, in citations with the same date. If so, that's the cause, and the disambiguation behavior will vary depending on the style used.

If you want to change the behavior, you can modify the style to produce the form you prefer, or you can stick with what the style is giving you, which is probably correct according to the style guide's rules. Post back if you need help with modifying the style.