I have seen rifles fire from just closing the bolt wrong or too fast.Not sure what causes this
Not a RARE occurance on bench guns. Usually from 3 causes:
1. Accounting for proabley 99% is a Trigger/Sear adjustment that was done a LITTLE to light ( for the shooter ) or one done BADLEY that can happen to anyone. That is the reason for the way a line is ran on the Modified guns. NO rifle goes on the bench with ANYONE FORWARD of the benches. Even when the command is given to bench the weapon NO BOLTS OR MAGS ARE ALLOWED. Once a weapon is on the bench, in rests or bags IT CANNOT BE MOVED out of that position ( except for elevation/windage adjustments ) Once the bolt is allowed to be inserted the line is CALLED HOT. If for some reason a cease fire is called they still cannot be taken out of the rests/bags. The command to DROP MAGS, REMOVE BOLTS, and depending on the range, chamber flags inserted. All shooters are then required to move away from the bench usually behind a marked linel. Not until these conditions are met and verified by referees/range safety officer(s) WILL ANYONE GO FORWARD OF THE BENCHS OR REMOVE A WEAPON.
2. Very, Very seldom a mechanical malfunction occurs that causes the firing pin to either slip or protrude far enough to contact the primer.
3. This happens with centerfire guns where a primer is NOT SEATED to the correct depth and the bolt face on closing will detonate it. This is why you don't want to sub Large Rifle Primers for Large Pistol Primers. The rifle primers although the same diamiater are taller and wont seat the proper depth.
If you watch the video in the first slam fire watch how the young kid SLAMS THE BOLT HOME. You can see the rifle move forward on the bags and contact the fore end stop on the front rest as he slams bolt home. In later frames you can see him properly position the rife in the rest and move it forward until it contacts the stop. After the first one I imagine the adult said something to him about it. Bottom line a young shooter NOT being tutored very well. I would think the gun has a very light pull. A lot of the unlimited guns have triggers down to 2 OZ and less. The bolt actions HAVE TO BE SLICKER THAN SNAKE SNOT on these guns. You should be able to work the bolts with just one finger, open, extract, and close. The rifle is NOT FITTED TO HIM thus causing him to OVERWORK IT. Again BAD instruction/teaching.