I've had a similar experience with my last two beers. I used yeast from a former beer (primary) for both of them. I was going to dump one out in fact, but I thought I'd try it again and it seems to be mellowing. Did you use fresh yeast, or cultivated yeast from another fermentation?
The only thing I can think is that for mine at least, the reused yeast gave it a stronger yeasty flavor somehow, and it took much more time to fully ferment.