What year is the trailer? Older Jayco PUPs had some issues with the frames flexing to the point that they actually cracked over time. The flex point is in front of the wheels where the door is because there is no solid wall there to support the frame. If your frame is flexing and I suspect it is, as the trailer is bouncing up and down on the road the wall and door can easily separate enough to allow the door to open. On one PUP we had I put a latch on the outside of the door to prevent this from happening.
2013 Jayco Jayfeather X20 E, 2008 GMC Sierra 1500 Crew Cab Z71 SLE