I'm having the same issue and have been for over a month now! I won't even be near my phone, or say anything relatively close to anything that would open Bixby up, and low and behold, it opens up! Highly irritating and wish I could find a fix for this.
I would like to look into this issue further for you. Let's look at a couple of things first.
Have you made sure that your phone is up to date on its software?
Have you tried clearing cache partition on your phone?