I believe that Tesla thought it's best value per dollar truck was the $40k rear drive model. The Tesla rear drive truck would handle better than rear drive ICE trucks because of the distribution of the battery weight. So they might well have thought that this would be the most popular model.

However when the pre-orders came in the rear drive model was much less popular than the other two models and so they rearranged their production schedule. This just makes sense given the market reaction.