Here’s one way to look at PPR as an aging issue, not as a malfunction:
The Volt switches from Electric to Extended Range Mode when the battery state of charge drops to ~20%, just a tad above the bottom "hard floor" level of ~15%. A normally operating 2013 Volt, with its 16.5 kWh battery, thus has only ~0.825 kWh of power in this 5% buffer for the car to "borrow" when generator output is insufficient to meet needs. THIS BUFFER IS ALSO THE SOURCE OF POWER USED BY MGA TO RESTART THE ENGINE AFTER THE CAR HAS BEEN TURNED OFF WHILE DRIVING WITH A FULLY DEPLETED BATTERY.
When the Volt is in good working condition, in most cases this small buffer is sufficient, and one is only encouraged to increase this buffer to prevent Propulsion Power Reduced episodes by switching to Mountain Mode before heading into driving conditions that may produce higher power demands.
Your Volt’s 8 yr/100,000 mile battery warranty says this: "Depending on use, the battery may degrade as little as 10% to as much as 30% of capacity over the warranty period" (which seems to have been increased to 10% to 40% for the Gen 2 Volt and Bolt).
"Acceptable" degradation over time impacts not only your Volt’s ev range and kWh Used per charge, but also how much power is being maintained in the 5% fully depleted battery buffer. The borrowable Extended Range Mode buffer power in a 2013 Volt may thus be susceptible to a drop from 0.825 kWh to 0.7 kWh or even 0.6 kWh while remaining within acceptable warranty limits.
If your early model Gen 1 Volt’s battery has experienced even a modest amount of degradation over time, conditions under which a Propulsion Power Reduced episode may be triggered may be encountered more frequently because less "borrowable" power is being maintained in the Extended Range Mode battery buffer.
Also more likely is that when you turn your car off during a drive (to go to a store, etc.), if the engine is running at that moment, the battery is being recharged to replace some borrowed power, and the soc is still below the "switch to gas" ~20% state of charge. When you then return to the car and try to start it, the charge in the battery is too close to the "hard floor" level, resulting in a Propulsion Power Reduced episode.
As Hellsop suggests above, before you turn off your Volt, wait until the engine stops running, which would then indicate the buffer is back up to the appropriate soc level. If your battery is a bit degraded from old age, that "topping off before shutting off" might be enough to provide adequate starting power without triggering the PPR.
One could, I suppose, counteract any reduction in the Gen 1 Extended Range depleted battery buffer size whenever one is anticipating driving beyond battery range by switching to Mountain Mode before the remaining battery power drops to the ~4 bar level (switching to MM when you first start driving is easier than remembering to do it later). This method will maintain sufficient charge in the battery to avoid PPR episodes when the car is turned off, and then started again a short time later.
If the vehicle is switched into MM before the battery SOC has dropped below the MM-maintained buffer level, all gas used will be used to generate power to propel the car and there will be no extra gas burned to recharge the battery. Then, when you are confident the car will be recharged before it is next turned off and then restarted, the car can be switched back from MM to Normal, and those ~4 bars of power (~14 ev miles) can be used during the final legs of the trip.
