Prop fired during sump/tank gurgle

Really need help here with my 80hp.  During this morning’s oil sump vent/gurgle, after counting 23 compression cycles, the oil tank gurgled.  Then I turned the prop to get one more gurgle and the prop kicked like it was going to start hitting the back of my hand.  ALWAYS TREAT A PROP AS IF IT WERE HOT!   But the question is how could it do this?  Do 912s do this?  Hand blue and hurting, I started the engine and did about 50 mag checks.  Each individually toggle switch throw dropped the RPM about 175 + - nothing (0) differnce between one and the other.  And the engine shut down with the switches as it should, but I did hear one backfire as it stopped.  What caused the prop to fire?  Is this normal?  Is it something I can fix?  I run 100% mogas with ethanol but previous owners had run 100LL.  I replaced the plugs 30 hours ago.  The engine had not been run in over 24 hours, so it was stone cold.    In my 230 hours with a Rotax 912, I have never had the prop kick during hand rotation.  I didn’t think our engines could be hand-propped to fire?  HELP!