Sherry is definitely due to oxidation and it can appear relatively quickly if the the beer is very oxidized and/or stored very warm.
The sweetness might be something that was always there, but which is becoming more obvious as hop character drops, which is also a sign of aging.
My guess is that you have an oxidation problem and that it's severe enough that the beer went from being "papery" and "dull" to sherry-like without you noticing it.
You're lucky that this just happened to a batch of big beer where sherry-like notes aren't out of place. As Euge said, it might work well as a Stock Ale or Old Ale.
In the future, minimize aeration during all steps of the brewing processs - from mashing in to bottling. Be very careful about splashing or aerating your wort during the hot side - mashing or wort transfer to the boil kettle. Boiling will drive off oxygen, so you don't have to worry so much about the beer during that phase. Likewise, any oxygen you add at yeast pitch will also get scavenged up by the yeast pretty quickly, so that's not a problem either, but after that, you have to worry about oxygen pickup again. If you're conditioning in secondary for long periods of time, consider blanketing your beer with a bit of CO2. Other folks have given suggestions for minimizing aeration during bottling.
Once your beer is bottled and carbonated to the level you want, the next thing you have to worry about is heat. If you can keep your beer at near 32 *F, it can remain brewery-fresh for months. (Commercial beer stored at near freezing temperatures can stay in near perfect condition for as long as 2 years.) If you can't keep your beer in cold storage, keep it as cold as possible - the warmer the beer, the faster it ages. For commercial beer, the shelf-life is about 100 days. For HB, the shelf-like can be even shorter.
Excessive heat can kill your beer fast. Fermentation scientists deliberately introduce oxidation in beer by holding it at pasteurization temperatures (120-160 *F) for 2-3 days. Storing beer in warm conditions, like a hot attic, or next to a radiator, can severely oxidize it in just a few weeks.