The slide stop spring acts to hold the stop in the down position unless it is being overcome either by the pressure from the follower in an empty magazine or the pressure from your thumb on the underside of the stop.
The larger grip angle of the Steyr may be a factor in this phenomenon; when you slam the loaded mag home there is a larger component of force acting to push the gripframe forwards. The inertia of the slide will tend to hold it in its present location as the gripframe is being pushed forwards, thereby allowing the slide stop to lose its grip on the slide.
BTW, with my M9, if you insert the loaded mag gently the open slide stays put. If you insert it with gusto the slide will spontaneously go into battery.
The larger grip angle of the Steyr may be a factor in this phenomenon; when you slam the loaded mag home there is a larger component of force acting to push the gripframe forwards. The inertia of the slide will tend to hold it in its present location as the gripframe is being pushed forwards, thereby allowing the slide stop to lose its grip on the slide.
BTW, with my M9, if you insert the loaded mag gently the open slide stays put. If you insert it with gusto the slide will spontaneously go into battery.
