It's possible that the bulk of fermentation is complete, and the beer has moved in to the conditioning phase. During the conditioning phase, diacetyl is consumed by yeast and transformed into flavorless compounds. One byproduct of this process is hydrogen sulfur gas.
As to why you may have more diacetyl in the first place, there are two parts of your process that immediately jump out to me. The first is temperature. While you're in the range Wyeast lists, I think you'll find you're a little higher than most people start that yeast. Second is the possibility that you have a larger amount of petite mutants in your yeast--either from the number of generations, or from something more specific to your process (crash temp on prior batch, or fermentation temps).
There are numerous threads hereabouts on rhino fart yeast, and other terms for sulfurous smells coming out of the fermenter. You can look for those for more insight. If, as I suspect, you're already (at least partially) in the conditioning phase, you will need to just let it ride out and clean up.