When I asked this question I was told it's flavor gnomes.
Currently I have several in my collection that did this when I used them. They've now been washed 2 to 3 times and are still going strong, so it's not necessarily a sign that they're going out. The only trouble I ever had with them is once in a while they would pop and at the same time spit a small amount of hot
juice out the mouthpiece. That made me wonder if maybe the ones that pop don't have their coil completely covered or something. It might be my imagination, but the poppers also seem to produce better vapor.