A hang fire can be caused by several things...
In military ammunition improper storage , heat humidity handling can cause the priming compound and the powder to deteriorate over time, leading to inconsistent ignition.
Wartime ammunition produced under pressure to get it into the hands of the troops, can lead to reduced quality control in the name of productivity. Smaller quantities of primer compound in the primer cup, excessive force when seating primers can lead to the shattering of the compound and causing it to gather at the lowest point when the rifle is shouldered. Thicker primer cups, light strikes from the firing pin.
Excessive vibration is not good for ammunition as it can cause the powder to change form, by rubbing against each individual grain with in the case or having the fine residue gather in the base of the case. this too can cause inconsistent performance.
Sometimes the priming compound itself is unstable and begins to chemically degrade and do so in a relatively short time, the chemicals used in powder production can if they begin to break down attack the priming compound, you also have outside contamination by moisture or oils during manufacture.
Then there is the case itself, deep primer pockets, smaller or deeper anvil in Berdan cases, off set flash holes.
Then you have the chance that it was an outright bad lot of ammunition that made it out into the market, due to the test lots drawn from within this particular batch being of sufficient quality to meet the minimum requirements.