I see you're not having that problem on the rest of the sock. It must have something to do with your decrease not being worked tight enough. I had that happen when working a decrease at the very beginning of a new needle. Wasn't a problem doing the decrease at the end of a needle, just at the beginning, for me anyway. Because you already have to work it tight, being on a new needle as EK is referring to, but then you've got even more of a potential gap, due to the decrease. You need to tighten the stitch as if it's stretching to the second stitch on the new needle, because really that's what it has to do, when you've decreased the first stitch. Hope that makes sense!
Was the decrease being worked at the beginning of a needle by any chance? That would definitely increase the likelihood of it happening; in which case you just need to tighten up that decrease in this circumstance. If it's not at the beginning of a needle, it must just be the way you're working the decrease in general, and you need to be conscious of tightening it up in general.
Hope that helps!