massive battery drain on all tags at the same time

[alas...@gmail.com]

I have encountered a interesting (and annoying) phenomenon with my tags.  Out of the blue, the voltage level drops significantly on all my tags at the same time (regardless of battery age).  Some of the tags die and the few that live suffer on.  Eventually, the voltages go back up, but never to the level they were at before the event occurred.  I have attached a plot of several tag voltages.  Notice how constant the voltages were before the event.  This has happened once before, and I ended up having to replace half the tags' batteries, the other half survived, but the voltages were significantly lower then when the event occurred, significantly decreasing the battery life. 

My question to the group is, has anyone else observed this event with their tags? Any possible explanation?  I have no doubt I will be replacing the batteries in many of my tags after this is over.

Thanks