Sulfur compounds are common during fermentation but they tend to blow off quickly with age. The off-flavor skunk is usually reserved for UV exposure but maybe what you're getting is more like the brimstone/rotten egg/rhino fart. Primary considerations are yeast health and temperature.
If this persists through an inordinate amount of time, I've heard of folks gassing off their bottles a tad to get rid of compounds in the head space. I can't corroborate since I purge in kegs (much easier), so while I'm not suggesting you do this, maybe look into it.