Assuming the packs were not dropped or damaged mechanically, shorts often occur at the cell tabs/terminations
It is generally a good idea to strain relief the balance wires (attach them to the dischsrge leads with tie wraps for example)
Also do not hang or remove the packs by the dischsrge leads (see people do it all the time)
It also helps to never charge past 4.1V and avoid discharges below 3.6 if possible
After that it comes down to internal manufacturing defects and luck !