Correlation vs. causality in other-triggered upward lightning in tower flashes

Upward lightning flashes from towers are sometimes preceded by other lightning activity in the vicinity of the tower. This observation has led to a classification of upward tower lightning into self-initiated, which are not preceded by nearby lightning activity, and other-triggered, for which one or more CG or IC flashes occur within a given preceding interval and within a given distance from the tower. The causality relation between other-triggered flashes and the preceding activity has not been established. In this paper, we hypothesize that at least some of the activity prior to other-triggered tower flashes can be explained as being due to chance rather than causality.