WHen I batch sparged, I tended to be a bit of a heavy stirrer. I also just dumped my sparge water in rather aggressively, Something I've recently read may contribute to oxidation. Maybe I'm dealing with a combination of the two?
I doubt this, highly...
I think it could be oxidation post fermentation and possibly at bottling time.
I'm dealing with a beer, a rye pale ale, that had a slight butterscotch happening in the keg. It may have been oxidation from not adequately purging the headspace. It was 3.5 gallons in the keg, perhaps I didn't purge well enough. But I bottled a bunch of it off and noticed the butterscotch last night. No slick mouth feel though. I don't know where else it would come from (I am damn sure at this point that it's not coming from my gas lines).
Then again, I could be imaging it. It was with Kohatu hops, rye, 4oz special B, and 4oz Victory. It could be a combination of those malts that makes me think butterscotch when it could just be caramel/toffee that I'm tasting.
It's not too unpleasant either, in this case. But I've definitely had some IPAs ruined by contamination of my gas lines (mold in disconnect from back flow) that caused intense butterscotch after a week or so in the keg. I almost quit brewing because I couldn't figure out why that was happening.
So I'm usually put off when ever I perceive what might be butterscotch, anymore...