The lift bars swing up over center, and should therefor wedge into place, using the weight of the roof and tension of the liner to hold. There is a variety of reasons that they could come lose - angled parking, side to side movement in the camper, light weight roof, driving, and so on.
an easy to install security strap kit is available for the Phoenix Simple Lift..............
https://www.youtube.com/watch?v=8Io8xXfbXxc&feature=youtu.be
Instructions for ordering yours are included in the comment section of the page.