This also depends on the dog. I find when I pull my dog in a trailer he tends to move towards the front of the trailer. Maybe this is since when the dog is in front of the trailer axle more weight is on the rear axle of the bike and the trailer feels more stablefor the dog (he do not like to sit on something that fells wobbely). Therefor I put some of my lugage in the front of the trailer (light stuff like sleepingmat and sleeping bag) so that he is forced to sit further back. I think the weight distribution on the trailer is the main issue. Also try to put more weight up front and less on the rear wheel.