A trailer jackknife can happen when you brake the tractor, and the trailer’s momentum pushes the back of your tractor around. With a bike trailer, the bike is the tractor.
On a seatpost hitch trailer like the Burley Travoy, jackknifing should be very difficult because the trailer is hitched forward of the rear wheel. I did manage this trick, however, by braking in a turn on a sand covered trail. I recovered quickly and didn’t fall, but it’s something to be aware of.
I loaded the Burley Travoy up with 30 pounds and tried to force a jackknife. I barely managed to lift my rear wheel only when I braked hard from over 20 mph. In retrospect, I probably should have tried turning more sharply in the turns, but since I didn’t actually succeed in jackknifing the trailer on camera I think it’s safe to say the Burley Travoy trailer is stable enough for general, non-racing cycling.
The jackknifing tests begin about 20 seconds into the following video.