Top reasons for non-spinningness:
1) Too windy. Chances are the past few days it's been well over 30m/s wind speed in some areas, so the turbines shut down and feather the blades to avoid structural damage. They will normally still turn to face the wind though.
2) Grid work/maintenance. Would require whole park shut down for that.
3) Faults or service. Recognisable by just one or two being off usually.
They don't turn them off after having met some "quota", they keep producing, but whether the power produced gets used is all down to the dizzying quota and purchasing system the energy companys and government have. As some articles have mentioned, renewable schemes can get paid for "unused" energy and load balancing, but it's all contract stuff and I'm sure Chris knows more about that side, hatred notwithstanding!
If the average business park turbine is only 50-100kw, then I would say yes, they are pointlessly small! One of mine is 5MW, and we have 30.