not a big deal...
Hey, not to worry. I've had several automatics do this when I slam a fresh magazine home. It just means that you are jarring the frame hard enough to cause the slide to move just a little, and since the magazine is loaded, the slide stop gets disengaged. The fact that the slide stop gets disengaged lets the slide go into battery. The gun is now "hot" and ready to fire.
Actually, it's kind of a handy thing, if you think about it. It saves you the time of racking the slide or disengaging the slide release before you can fire again. You just have to be aware that once the slide is in battery, you are ready to rock n' roll. But, as always, the gun won't fire unless the trigger is pulled.