In general, my experience has been (and from what I've heard from others' dogs) that boxers do tend to have more sensitive tummies (compared to the other breeds that we've owned over the course of my life). My first boxer was a mess with regard to her tummy issues. My second boxer can eat nails; you can't intentionally give her the runs. My third does well as long as we don't over feed her.
Other things that I've done to minimize the runs are to feed smaller meals more often (we feed three times per day), feed a high quality diet (we feed raw), keep bulk fiber on hand in the event of the runs (I used to use canned pumpkin until I found a product called "Olewo" carrots. They're dried pelletized carrots that are imported from Germany. I order online, I think they cost approx. $11.00 for a 2lb box.) I've also added the digestive enzyme/probiotic combo that I mentioned above.
It also could be a virus. Sometimes things just happen and they need to run their course.
The runs happen on a very rare occasion here any more, but as I said before, my first dog (my backyard bred girlie) was a mess so I've just learned to have these things on hand just in case.