I think the most common design safety factor used for retaining wall stability is 1.5, both for overturning and sliding. What is the basis of your question?
i thought 2.0 for overturn and 1.5 for sliding was the most typical. i hadn't ever thought about why before... and don't have a wild speculation to offer yet .... maybe next month or after a few more beers.
There is a potential for more stress distribution in sliding. Further, overturning is usually a "catastrophic" event, whereas sliding is generally not.