I was on the SF34 at a regional/have 8000 ish hours in it total (both seats). We didn't use those straps as a matter of SOP. I do know that on the GE CT7-5A2 engine used on the A model, a mechanic could hold the prop stationary while you started it due to it being a free turbine. Therefore I could see this possibly happening if someone did a really really sh*tty walk around plus not looking out of the cockpit on that side plus etc etc etc. Who knows, maybe this company used the prop brake feature (ours did not) as a matter of SOP, so that prop wasn't supposed to be turning on taxi out. Weird things happen in this business when the stars all align in exactly the wrong way on a given day.