Reloads or factory ammunition?
What primers on the rounds?
Primers are unpredictable things, they are made so that they ignite between “all fire” and “none fire” limits. The limits determined by dropping a steel ball on to a primer. All fire limits are around 15” drop height, I forget the none fire, might be 4 inches. However a primer lot is accepted even if primers fire at the lower limit, just too many primers cannot fire at the lower limit.
I am unfamiliar with the Savage 99 action, it is unusual for a firing pin in a manually operated arm to develop enough kinetic energy to ignite a primer. It is very uncommon in semi auto’s with free floating firing pins. Based on data from William Davis’s report on M16 slamfires, and his calculations, it appeared that a 1:64 million chance of a slamfire in the M16 was considered acceptable. About five to six people in the US die from snake bites, so dying from snake bite is around a 1:50 million chance in the US, so while both are rare, they will happen.
Incidentally, I heard 30,000 people a year die from snake bites in India!
It is possible a high primer could slamfire, given a hard enough hit, and several other conditions have to be met. According to CCI high primers are the most common reason for a misfire. The anvil has to be supported from the bottom, can’t be hanging in the air, and it has to be pushed into the primer cake. However, lets say that you have cocked primers, part of the anvil supported in the primer bottom, and some of the primer above the case head, I believe a hard strike could set off a sensitive primer, but I don’t have any evidence to prove so. If the primer pocket is shallow, primer above the case head, anvil firmly seaedl and pushed into the primer cake, a hard strike will set off the primer.
I suspect a mechanical explanation more than a chemical kinetic explanation. You really should check to see that the hammer is not following due to sear issues and the firing pin and hole are clean.
You should check this out as not all slamfires are in battery.
If the ammunition is fine and the rifle mechanically correct, maybe your big finger tripped the trigger.
If I'm not shooting, I'm reloading.