So JGWeissman explained why we describe light as a wave. The behavior of light can be accurately modeled with a certain kind of equation, this is the same kind of equation we use to describe traditional waves. But that doesn't seem to answer byrnema's question "What causes them to oscillate?". Some interpretations won't even leave us with waves and I don't know if there can be causal histories of wave oscillation when the wave doesn't have a medium. I guess you could generate an EM wave and go "Look!" but it seemed like byrnema was asking something like "why does light move like a wave" which is more or less unanswerable right now and probably will be for some time.