Out of 195 separate e-cigarette fire and explosion incidents in the U.S. that were reported and that they studied,
from January 2009 to Dec 31, 2016 here were the causes,
61 occurred when either the device or spare batteries for the device were in a pocket.
60 occurred while the device was being used.
48 incidents occurred while the battery in the device was being charged.
18 incidents occurred while the device or battery was stored.
7 incidents, it is not reported whether the e-cigarette was in use, stored, or
being charged.
1 incident occurred during transportation on a cargo aircraft.
I work in a technical field with electronics and will not use a mech either.
That breakdown would have been more useful it it broke down device type: mechanical, internal battery regulated, replaceable battery regulated, etc.