Proto-Oceanic had a transitive verb *suRi ‘follow, be in motion behind somebody/something’, ‘accompany’. This verb underwent several grammaticalization processes. It developed into a preposition with the sense ‘(motion) in the direction of X’, a preposition whose meaning can be broadly characterized as ‘concerning’, including the marking of locutional topics, and into a reason/cause preposition and clause marker. After discussion of the semantic aspects of *suRi and its reflexes in various Oceanic languages, the paper focuses on the development of the reason/cause marking functions. It is argued that the factor that motivated the development was the post hoc ergo propter hoc fallacy: because state of affairs B temporally follows state of affairs A, it may be seen as being caused by it. There is also evidence of independent developments from the meaning ‘follow’ to a cause and/or reason marking function in other languages with different etyma involved.