Let's assume that, with the work they had done to keep clear of Vanmarcke and Van Avermaet, Terpstra and Vandenbergh were destined to come 3rd and 4th. Once Boonen had manoeuvred himself into 4th wheel with about 8km remaining, that seemed inevitable, and Etixx looked to have no ambition of a 1-2 by that point.
So why did that not happen? I have not done as much reading of obscure magazines in Flemish as some of you, and claim no knowledge of his mindset or personality, but that attempted escape at 5km to go looked to me like pure hubris: "I could simply beat him in a 250m sprint, but I want to have my head up and my arms raised for those last 250m and to take this in style."
When that failed, Terpstra seemed to recognise that the plan had changed, and was ready to exhaust Stannard with multiple chases for the remaining distance. Vandenbergh, I suspect, rather than thinking "anyone other than Terpstra" as has been suggested, was still operating on plan A and underestimated how much the failed attack had taken out of Boonen. As far as he was concerned, his job was still to operate a two-man derny so that Boonen could win the sprint, and he was probably wondering why Terpstra had pushed quite so hard.
Stannard did well to take advantage of the confusion and had the strength to bring it down to one on one by the end, but still needed a third Etixx mistake, that of Terpstra either totally forgetting last year's finish in the same race (Stannard can outsprint a potato after all) or thinking that he is better at a long sprint than Van Avermaet. Surely Stannard could not believe his luck when Terpstra took over the lead as early as he did.
Great calm head, and great strength by Stannard, but he still could not have won without the Vanmarcke/Van Avermaet threat behind, and the pride of Boonen/Terpstra. And the lack of race radio to tell Vandenbergh that the situation and plan had changed.