There is no reason to make an assumption, hence there is no "safest" assumption in this case.
Of course there is a reason to make an assumption. The reason being to aid you in deciding what to do next. For my part I am going to assume for the time being that this was a deliberate act. As a result of my assumption I am going to be staying off of planes for the forseeable future. I suspect most people with brains in their heads will error on the side of caution with me and stay off of planes as well. If they have a choice of course.
DON'T SAY THAT! That will mean that folks who want to believe that this must be due to a bomb and/or government cover-up without getting more information will have the rug pulled out from under them.