Question: Considering that there are many different theories about how time travel works, such as the grandfather paradox, temporal paradox, etc., why did Adam assume that the butterfly effect was the primary rule of time traveling?
Answer: It's just the rule they used for the movie; not a lot of thought was put into the time-traveling rules with the obvious inconsistencies the movie has throughout regarding it, especially in the ending.
Chosen answer: The James Bond film "A View To A Kill" came out the year before in 1985. In the film, Bond's snowscooter is blown up by the Soviets, so he takes a wrecked ski from it and improvises it into a snowboard. Snowboards existed at the time, however, it wasn't until that film that their existence became well known. One year later in 1986, they'd still be very rare, but it's likely that his wasn't the first that the ski patrol had seen.
Captain Defenestrator