The majority of sliding screen doors have rollers (wheels) that ride upon a raised track rail and the door itself is merely held onto the track rail with gravity. Occasionally, there may be some spring-loaded rollers or devices at the top of the screen door to provide additional downward pressure.
If the screen keeps popping off the track, it usually needs new rollers. However, there is another common situation which causes the screen to pop off… even when the rollers are new.
The stationary panel of the glass patio door is designed to be securely attached to the jamb and be parallel to the screen track. The glass stationary door is usually held in position by brackets at the edge of the door.
If the bottom bracket is missing, broken or loose, the stationary door, over the years, can creep outward enough to encroach upon the path of the sliding screen. Then, every time the screen moves past this point, it hits the bottom of the glass stationary door and pops off.
Solution: Push the stationary door back into position so it will not interfere with the screen door and properly secure it with a bracket.