I misread the link bug report. Here is what is happening
I drag a selection with the deconstruction planner by holding the mouse button.
Now without releasing the mouse button hold shift and the selection switches to cancel construction/deconstruction mode but the selection isn't confirmed yet because you are still holding the mouse button.
Ok release the shift key and now hold the control key.
Oops. Better hurry. your stuff is getting deconstructed....but...but you never released the mouse button you say?
That's the issue.
I started a big deconstruction on my base because I hit control by accident. Hitting control key while dragging the deconstruction tool starts deconstruction.

[edit]
Misread the linked bug report.