Based on the episode synopsis for both "The Flash" and "Arrow" regarding their respective crossover episodes, it seems like both events are triggered because Oliver needs Barry's help, as opposed to the other way around.
I remember the show's producers saying in a interview awhile back on how they were going to be conscious about making sure that they create situations in the future where the obvious answer wouldn't be for just either Oliver or Barry to call up each other for help to deal with one of their threats. If they still remember that, I hope they keep to it since I could think of a lot of scenarios in just "Arrow" alone where Barry's gifts would have been useful.
Hell, had Barry been present during the battle against Slade's army, they would have all been defeated by Barry with him just using super speed to stab them with the cure.
