hmm what's happening in the middle of the bridge then...? because a bridge will typically be mogulled up from one end to the other. I'd imagine both reasons are true.... it's just that recently I was following a couple cautious riders and they both locked them up when they got to the bridges even though they were only going 30 to begin with....LOL
Moguls are contagious. Once a few are started, the suspensions these days compress and then rebound. That action, along with power to the track causes the sleds to dig their own. So it is a combination of both stated above, but the nailing after just tosses loose snow onto the crossing, which gets bumped up much quicker than a groomed/packed trail. We are our own worst enemy when it comes to moguls on the trails!