Excel 2003

Sheet1 has one combo box with ListFillRange from a couple of cells  in
Sheet2.

Drag and drop (or ctrl-x/ctrl-v) of any cell in Sheet2 will cause the change
event to fire.

Sounds like slightly eccentric behaviour in Excel.

It'll go away once you stop moving the cells in the constants sheet.
(Patient: "It only hurts when I do this. "Doctor: "So, stop doing that.")