How can a used alkaline cell rate at 1.9 volts?
Battery tests are generally done under a light load. If your meters are 10MOhm and the measurement affects the battery, it will likely drop much more notably with a 1kOhm resistor attached (which is only 1.9mA assuming 1.9V), let alone 100 Ohm. 

If you want to use voltage as an indicator for battery condition you need to make sure you have a small current loading them, or the measurement has much less meaning than you think.

However, it is peculiar, regardless for an alkaline 1.5V-rated battery to become 1.9V. It is possible to get them to that voltage, so long as they are not 100% empty, for a short while (range of seconds to minutes, maybe a bit longer) without physical damage.
But it takes care and attention, which is unlikely to have been applied in this case in an inadvertent manner.

There may have been something strange going on in your device, which may have done something silly to the weakest battery in the whole chain. But if it stays above 1.6V even with 100 Ohm loading, I'm betting it is in fact not an actual Alkaline cell, as Plasma hinted at in the comments.