In dirt, the trail often goes negative. If you drop a plumb bob down the axle line you will find that the contact patch for the tire on one side of the axle is equal to the other side. The plumb bob turns out to be the centerline of the contact patch. If you are calculating the trail you often start with the center of the 3-4" contact patch as one marker and the second marker would be where the rake centerline crosses the pavement. Join the two and that is the trail.
If you think about riding your bike in a paved parking lot your trail will be fine and provide good self centering. If you dive off the pavement into sand, for example, things are OK for a second, but as the bike sinks into the sand the steering gets heavy. Then the handlebars start kicking to one side or the other. What has happened is the contact patch in the sand has gotten very, very long. As you move ahead there is a bow-wave of sand moving upwards in front of the tire. If you measure your contact patch in the sand it might be 12-15". When you cut that in half to get the centerline it may be well beyond the rake line making the bike have negative trail. Negative trail means the fork is is trying to turn 180 degrees or reverse direction. That is why the steering goes wobbly in sand and you have to keep forcing the bars straight.
This happens in dirt also to a lesser degree, but if you run your front wheel up an incline or bump there will be a point where the wheel is "cupped" into the bump. In other words, the back half of the contact patch is on the flat and very short, but the front part of the contact patch is running up the incline and very long. The steeper the bump or incline the less trail you have. So, the trail varies wildly in loose and bumpy surface conditions.
The solo position could be a simple mistake or the rider might feel that the stronger self centering is better than the lack of turning ease because the speeds are slow and he can use the throttle and sidecar momentum to steer left and right (along with some muscles).
David