They also moved from a toroidal SMPS to an integrated PMIC for voltage regulation - but that was likely for cost/weight reasons but also hints at being able to more accurately measure and limit the charge or discharge current as a function of the cell temperature. This design is typically less efficient (generates more heat for the same output) so that strongly hints that the overheating issue was not from the pcb/electronics directly and from the cells themselves.
The addition of what I assume is a temperature sensor on the cell in the new design also points to the addition of some charge/discharge regulation based on the active cell temperature that wasn't present in the original model.
Moving from busbar to insulated leads also suggests some recurring issues with bus bar short/contacts but was probably not the cause of the overheating issue as busbars usually have lower resistance and higher current capacity.
If I had to guess I would say the recall was very likely due to cell overheating on high discharge (PD in/out) in high temperature or high insulation (in a backpack or in the sun) situations where there was no thermal feedback from the cell to the controller and the cell was outside of the specification temperature. Thermal runaway is a known issue with lipo cells and can cause cell rupture and combustion.
I've had tons of other packs that have gotten too hot to touch during high charge/discharge and that have "pillowed" up (there is an entire amusingly-named subreddit for shaming cells that have done this... /r/spicypillows) and that is without putting them in a backpack or other insulating environment, or doing so in high ambient temperature. I think anker is likely singled out in the recall because of their size and not necessarily that they made a worse powerbank than most others... but that is only anecdotal based on how many off-brand powerbanks I've seen fail in the same way (bluehive, inui and safuel specifically, to name names) without any recalls or sanctions.