In doing a few internet searches, doesn't seem like this is a very common occurrence at all. Like winning a lottery or getting struck by lightning, it is a possibility, but rather remote.
Although I would opt to keep my ereader in my carry on bag, I wouldn't be overly worried about placing it on a tray if asked.
Perhaps the OP can provide some more details on what happened:
Was the ereader in a bag or on a tray?
What airport did this happen in?
Was the ereader powered on at the time? In sleep mode? Or powered off?
Was the ereader in a case?
If it was in a case/cover, was it in a Kobo case or a third party case/cover?