Any idea why my thyroid replacement needs are going down? My levels were stable at 100 a year ago and now am being told 50 is possibly too much...

Underlying Grave's. I've had a few patients who were stable on a levothyroxine dose for hashimoto's thyroiditis, then suddenly requirements drop like this. Looking for an evolving grave's thyroiditis may help to explain the problem. For ablated patients, i've seen remnant tissue become hyperthyroid also.
Autonomous thyroid. Different thyroid preparations (eg synthroid (thyroxine) vs levothroid) can give different results, but not usually this much. It is possible that you have some functioning thyroid tissue left, and this tissue may be autonomous to some extent, i.e. Not shut off when you take the synthroid (thyroxine). It would be useful to know why you were started on thyroid replacement initially. Autoimmune thyroid? Recovery?