I tried using my fairly basic Paladin deck on casual mode to grind out a quest, left it out overnight, and when I woke up, the deck had a winrate of 3, yes 3% (2w/29l).
When I played a few games myself, I won 4 in a row without too much trouble, though I suppose those were at a lower level of play.
I was using the DeckRules_Default rules, which have worked well before, but somehow now has no synergy with the same deck.
Anyone have any idea what went tits up this time?

Decklist:
2x Avenge
1x Noble sacrifice
2xZombie chow
1xAcidic Swamp Oooze
1xArgen protector
2x Knife Juggler
1xDivine favor
1xMuster for battle
1xAldor Peacekeeper
2xShattered Sun Cleric
1xSpider Tank
2xTruesilver Champion
2xBlessing of Kings
2xConcecration
2xKeeper of Uldaman
2xMurloc Knight
1xMukla's champion
1xTuskarr Jouster
1xSylvanas Windrunner
1xDr.Boom
1xKel'Thuzad

I have run the exact same setting multiple times over the past 4 months (only when I have a pally quest) and have gotten 40-60% win rates. This was way different.
Like I said, unless the AI has had some overhauls with the last patch, this isn't the issue.
Again, this has worked pretty well before. This is the first time I have had a rate at this level.
My "elo" should have tanked as I lost again and again, so this shouldn't be the issue. It never started winning again, judging by the logs.

I'll try again tonight and report back. Just seemed like an extremely large statistical anomaly.

If you run the same deck for a very long time and the deck suddenly loose a lot, it could be due to meta change as well or playing against decks that counters your deck (certain decks have a very high winrate against other decks before drawing any cards, some match ups are very hard to begin with and require almost perfect draw and luck).