In my experience that happens in like 80% of Cantillon bottles, no matter how they're stored*. I'm surprised you haven't noticed it before, actually. In some of the older bottles it can get pretty gross, sometimes even black like it's moldy. Even when it's not bad it does smell spoiled, or something. But I have never noticed it actually affecting the taste of the beer. Most of the time I don't bother doing anything with it, but if it smells/looks bad then I mop it up with a napkin or something before removing the cork.
*I don't think storage here matters at all because Cantillon boxes (for the 750's, at least) are the kind that pack horizontally. And they're all stored horizontally at the brewery. So no matter how you got your Cantillon bottles, they were horizontal for a significant amount of time. If you got them from a US shelf, they were horizontal during the shipping process, so they were horizontal AND being agitated fairly aggressively. I think that if that leakage hasn't occurred by the time a bottle gets into the US, it won't occur from tame horizontal storage in a cellar. But I could be wrong.