What causes a guitar top to sink or belly?

Same villain as most acoustic guitar structural issues: sustained string tension, working against a top that's either lost some of its structural margin or was never quite built to handle it long-term.

Bellying near the bridge

This is the classic symptom — a slight dome or bulge in the top just behind the bridge, sometimes accompanied by the area in front of the bridge sinking slightly. What's happening is the string tension is essentially trying to rotate the bridge forward and pull the top up behind it, and over years, the wood (and the bracing supporting it) can start to give in to that constant pull.

Why it happens more on some guitars than others

Lighter bracing (including scalloped bracing, which trades some structural margin for tone), thinner tops, older guitars where the wood and glue joints have had decades to slowly creep, and guitars kept in poor humidity conditions are all more susceptible. Heavier string gauges and higher tunings accelerate it by simply adding more sustained force to the equation.

Is it dangerous?

Mild bellying is common on older guitars and isn't necessarily an emergency, but it's worth monitoring — action creeping upward over time is often the first sign you'll actually notice, since a bellying top changes the geometry between the strings and the fretboard. Severe bellying can be a sign the bracing itself has come loose or failed, which is a more serious structural repair.

What to do about it

Mild cases are sometimes managed rather than "fixed" — keeping humidity stable, not going heavier on string gauge than the guitar was designed for, and monitoring for progression. More significant bellying may need bracing repair or reinforcement. If your action has crept up over the years and you're not sure why, this is worth ruling in or out.