Guild Wars 2 is set 250 years after the events in the original Guild Wars. "Technology has advanced a great deal, and we wanted the engineer to personify this progress. And frankly, we didn't want to feel constrained by typical conventions as to what is or is not acceptable in a fantasy story," Buenaobra explains in the Engineer FAQ on the official website.
Now we know that the class fits with the timeline, but this does not explain the other issue fans have with this class. Why would the nature-loving Sylvari be interested in this type of technology? Buenaobra notes, "Players in general are exceptional and unusual figures, heroes who stand out among their people. Just as there are asura warriors and norn necromancers, it's not unheard of for an inquisitive, mechanically-inclined sylvari to become an engineer."
Check out the full FAQ on the Guild Wars 2 official site for answers to other important Engineer questions.