The main reason is he the combined internal impdance of two batteries.
Connecting 2 batteries in series make the total impeadance as a sum of
each so if the load consumes certain current the voltage will drop to
the unsustainable level by power supply of the unit.
The new alkaline battery has ~0.2ohms(good ones). The old one has
1ohms. So 2 new ones may have 0.4 and old and new ones 1.2. The new battery voltage is ~1.5and the old one is 1.2V (open circuits) so 3V/0.4=7.5A or 2.7/1.2=2.25A of short circuit currents.
There could be certain conditions of batteries internal impedance,
their voltages and load impedance that creates reverse voltage on older
battery resulting in quicker damaging that one. It is specially
possible with pulsed load. So as for your loads (pulsed one) it can be
more critical than for torch.