Slight parameter issue, which is absolutely the fault of our lack of YQL documentation and nothing I would have expected you to know. Try this instead:
SELECT * FROM fantasysports.players WHERE league_key="[league_key]" and sort="AR" and sort_type="season" and sort_season="2010"
The "sort_type" parameter deals with the type of coverage you're working with (season, date, week, etc), not the actual value you're sorting by. That's just "sort".