Why do screen splines keep popping out?

The frame survives here; the spline does not

Seminole County publishes an Ultimate Design Wind Speed of 129 mph for Risk Category I, the category the Florida Building Code permits screen enclosures to be designed to. That is a materially lighter wind load than a coastal county’s cage carries, and the county sits below the 140 mph wind-borne debris region threshold — Seminole County is not in the High-Velocity Hurricane Zone. The practical result: in a storm, the aluminium frame here usually survives intact and the mesh blows out of the spline channel first. That is a spline-and-mesh failure, not a frame failure, and it is why the standard post-storm job in this county is a rescreen rather than a rebuild.

Why old spline is the actual cause, more than the wind

Original spline on a 1980s–90s Lake Mary cage has had thirty-plus Central Florida summers to harden and shrink. Hardened spline sits proud of the channel instead of gripping down into it, so ordinary wind — not even storm wind — walks the mesh out a little at a time. Once a homeowner notices, it is usually because a whole edge has finally let go at once.

The correct fix clears the channel completely rather than rolling new spline over old residue, and sizes the replacement spline to the actual channel rather than to a default roll — see the spec table on the home page for the full standard a quote should meet.