You will need to determine if the machining was done correctly on the trigger guard and all parts surrounding it as well as replacing the trigger spring and possibly trigger guard plunger. Numerich currently has all of the parts that you may require in stock.
batj, make sure you read and understand the sticky, particularly the part about the dog bone shaped trigger guard pin.
As regards the trigger guard itself, how does it move currently? Does it hang up or otherwise stick, other than fully closed against the frame?
You would do well to take out the dogbone and examine the plunger and spring, and the dogbone itself for damage before ordering parts.
The most likely explanation is a weak trigger guard spring coupled with strong muzzle whip. As the gun moves upward the shooter's trigger finger pushes down on the inside of the trigger guard and opens it, which in turn allows the slide to recoil further rearward, bounce off its rails and go flying downrange. This was a problem with the much lighter TPH, and Walther incorporated an interlock to prevent the trigger guard from being opened unless the magazine was first removed. I've never heard of this in a PP-series pistol, though.
I'd start by replacing the trigger guard spring. However...
The trigger guard is essentially a big leaf spring designed to serve as a recoil buffer, so it must have some flexibility. With the slide off, and the trigger guard closed, there must be a minimum of .060” clearance between the rear face of the upright stud of the guard and the front face of the barrel boss, in order to provide room for the trigger guard to flex. It might be possible for the trigger guard to transmit enough recoil shock to the frame to bounce open, but I have never personally encountered it.
If at least .060" clearance is not there, I'd replace the trigger guard too.
Mike, I had noticed that with the TPH, and assumed they just wanted the magazine removed for safety's sake. That guard is much thinner and the guard spring less strong.
That is interesting.
As an aside, I was aware that the trigger guard was to act like a bit of a buffer in the PP series; IMHO, it's not a very good one. The deutschers were being too clever by half.