It's heading for a North Atlantic track. The S-Turn could simply be to adjust its position so when it got on the track it was spaced between other planes on that track. S-Turns are a way of doing this without flying in circles or losing too much altitude.
Or perhaps he made the first turn north a bit early, got re-routed back on his original track, and then made the turn slightly later. Either way, a pretty normal move, leaving an interesting contrail.
Here's the global context
Patrick Roddie told me about it. Although he might also have been pointing at the large amount of contrails there - perhaps not being familiar with North Atlantic Tracks. These are the Sept 19 tracks, probably quite similar to Sept 18.
Source: http://www.perkins-aviation.ch/weather/NorthAtlantic.html
Source: https://twitter.com/webbery/status/909816960321388545