Step 1) Open your hero panel (Push H), and navigate to your finishers. Do not select a finisher, but note which finisher you have selected.
Step 2) Return to the Previous menu
Your finisher has just been reset.
It seems that this occurs once for each time you open your hero panel, and will not happen again until you close your hero panel.
This also does not occur if you select a new finisher while viewing the finisher tab.