I've run into this problem a few times in the sketcher.
In today's case I draw a circle, then I draw a horizontal line through the center. Now I want to draw several lines at arbitrary angles to the first line. So I draw a second line and insert and angle dimension. I set that angle to whatever I feel like.
Then my horizontal reference line moves!
Where can I look up the "rules" for how Inventor decides which feature gets moved when I redimension a sketch?