Back in 14, I reported a couple related issues here: Script steps move out of order unexpectedly. Both issues were fixed at some point in 14 and/or 15, but it seems one of them is back in 16 (Windows 7):
Intentionally moving a step moves the wrong step:
1) Select a step in your script so that it is highlighted in blue;
2) Now grab another step below that step and slide it up somewhere. Grab and slide without pausing between the grab and the slide.
3) Chances are about 1 in 2 that as you are sliding you will notice that a completely different step has been highlighted in blue. And if you complete your slide and release, you will see that this other completely different step has now moved unexpectedly.