I have noticed soreness in the trapezoid muscles when I first started riding some years back. After that first time, the soreness never came back. For me it could be attributed to constant pounding my fork was taking from the terrain and my muscles were not used to it. I suppose I got use to it? Another issue I see with beginner mountain bikers, beside ill fitted bike, is when they are riding down hill, they're tense up a lot by gripping too hard, holding the brakes hard, etc. It a snowball effect to making their muscles in the neck hurt.
It is one of the blessings of wilderness life that it shows us how few things we need in order to be perfectly happy.-- Horace Kephart