Over the years I’ve repaired quite a few guitars where the bridge has come away from the soundboard.
The reason that they do, is usually because they weren’t made correctly in the first place. Sometimes there’s not enough glue applied, but often the bridge has been glued on after the top has been lacquered. That means relying on the lacquer to act as adhesive, with about 200 lbs of tension trying to pull it off! Sooner or later it will probably fail.
The bridge failure on this lovely little ukulele is unusual though: the area of the soundboard under the bridge had been cleared of lacquer (and recessed too) back to bare wood, so there should have been no problem with adhesion. But on this occasion, the recessed area was actually smaller than the footprint of the bridge, giving a small gap underneath most of the bridge! So hardly any glue was in contact with both surfaces and not surprising that it gave way. The solution was to very carefully enlarge the recessed area to match precisely the dimensions of the bridge and then re-glue it with clamps holding it in place while the glue set.
But now the good news. You may have a guitar where the bridge has started to come away from the top. It can look quite worrying. But as long as the ‘unglued’ part is no more than about 30% of the bridge area, it will probably be ok. To check, gently insert a thin feeler gauge or thin piece of paper between the bridge and the top. Then just check from time to time that it isn’t getting any bigger.