Usually failed or missing mid-span support, or joists and beams weakened by moisture and rot. Crawl space moisture is the underlying cause more often than the framing itself, which is why drying the space is part of the repair.
More detail
Bouncy or sloping floors over a crawl space have a short list of causes and they are usually visible once someone gets under there. What to look for. Support posts that have settled into the dirt because they were set on inadequate footings or on no footing at all, which is common in older construction. Wood posts in contact with soil, which rot from the bottom. A main beam that has sagged between supports, visible as a dip in the floor above. Joists with visible rot, insect damage, or fungal growth, particularly near the ends where they bear on the foundation. Joists that were notched or drilled for plumbing in a way that removed too much section. Moisture is the root cause behind most of it. the mix of pre-1940 brick and frame in the basin, mid-century ranches in the first-ring suburbs, and 1980s-onward subdivisions further out A crawl space with bare earth and no vapor barrier evaporates ground moisture continuously into the framing above it, and wood held above roughly 20 percent moisture content supports decay fungus. Vented crawl spaces in this climate make it worse in summer rather than better, because humid outdoor air condenses on cool surfaces. What the repair usually involves. New footings poured to bearing, adjustable steel posts set on them, beam replacement or sistering where members have failed, and sistered joists where individual members are compromised. Then the moisture control that keeps it from recurring: a sealed ground vapor barrier, closing the vents, and dehumidification. What to avoid. Jacking a floor level quickly. Structural lifting is done in small increments over weeks so framing, drywall, and finishes accommodate the movement. A floor brought level in an afternoon cracks everything above it. And fixing the framing without fixing the moisture, which returns the problem on a delay.