I agree with RVbuilder about landing technique (Post #302) but my understanding is that many of these incidents occur at slower speeds nearer the end of the landing when the nose wheel has dropped to the ground even with full back stick held.
I don't pretend to know all the answers, but I can imagine a possible cause for this....
A large # of the flip-over accidents that I have first hand info. about were situations were the pilot was getting short on runway remaining. Some of these were situations where the pilot was still in the air and forced the airplane on the ground (pretty much guaranteed to ruin your day). Many of the others were good landings (they resisted forcing the airplane on the ground and waited to touch down in a full nose up flair), but because of extra speed and/or miss judgment of a proper touch down point they ended up rolling a bit long on the runway and the flip happened near the end of the roll-out.
Think for a minute about the dynamics of this situation...
Airplane is still rolling moderately fast so the pilot is braking hard. Because of the vertical position of the C.G. relative to the main and nose wheels, this automatically shifts more weight than normal, onto the nose gear. If the pilot is holding full back stick (as he should be), this will help counter the pitching moment produced by the de-acceleration. But what if the situation is getting a bit tight and the pilot keeps the hard braking going right until the end. So there is a strong pitching moment throughout the entire roll-out, but as the speed is decreasing, the elevator authority is decreasing. This means that the weight load on the nose gear may be increasing during the entire roll-out.
An example of this we have all experienced is an emergency stop in a car. The front end dives towards the ground and we are pushed fwd against the belts. If it is a true panic stop in a vehicle with ABS, we will stay tight against the belts until the instant that the vehicle actually stops.
The exact same thing happens in the airplane. The pitching may actually be worse because of how tall the airplane sits up on the gear.
We have something the car doesn't have.... pitch control. This helps reduce the pitching moment... until its effectiveness is reduced because of low airspeed. Add into the situation a bad pump just before the airplane is about to stop (now very low elevator effectiveness, but highest amount of weight on the nose gear), which pitches the nose up and it slams back down with the original increased load caused by the hard stop + the acceleration added from the nose dropping, and we have a bad situation.
So you might say that not all of the accidents happen near the end of a runway. I agree.
What if, because it is a shorter grass runway (a lot of them are) a pilot decides to practice his short field landing skills (or maybe a bunch of his buddy's are watching) so he brakes hard even though he didn't really need to. (Refer to explanation above of what can happen if you brake hard to a complete stop). He is actually trying to see how short he can land (or impress his buddys), so he is going to brake hard
to a full stop right? He is not going to think, there, I got it slowed down to 30 kts pretty quick, and then stop braking.
Now he may have done everything right (right on the proper airspeed, and touched down at the very beginning of the runway), so that he will be stopped at about the mid point of a 2000 ft runway (actually pretty good performance for someone that doesn't do it all the time), but he didn't need too, and he may pay a hard price for it.
I can imagine scenarios (but still guesses, just like everyone else) that could explain many of the flip-over accidents. The explanation above I believe covers a lot of them, particularly if you also add in improper maint. (low nose tire pressure), excessive weight on nose gear, etc., to the mix. One thing I am sure of... we don't always hear all of the pertinent details of these accidents. I don't mean to imply that it is always intentional (though I think it sometimes is), but if we did know all the facts, I think most of us would have a very different opinion about what caused the accident.