Interesting. This happens to me on occassion. I always blow out any excess juice before I start using a carto, and it seems to happen even if I've been vaping the carto for a while. It never occured to me that maybe it happens when the battery is getting low, so I'll have to see if this is the case. I always thought that it was just a 'normal' thing since a certain amount of vapor condenses on the user end of the cartomizer and it would be very easy to 'draw' in the condensed vapor, especially if vaping with the ecig in a horizontal position.