First of all, the water softens the gypsum and it starts to loosen. If you don't take steps to dry the plaster and get rid of all this moisture, you'll quickly end up with dampness, which turns into mold. Gypsum board can also get wet due to conditions where it will absorb large amounts of moisture from the air. This can include constant high humidity caused by environmental conditions, or possibly by nearby standing water, combined with poor ventilation.
If the damage to your wall is excessive, call someone who knows how to repair water-damaged plaster walls. If you notice a brown spot on the plaster walls, or if there are bubbles or scales, it's likely that the plaster walls or ceilings have suffered some water damage. Before repairing the cast, make sure that the underlying cause of the water damage has been thoroughly and appropriately treated. Using a bucket of clean water and a sponge, wash the surrounding area as a final cleaning exercise and then allow the water-damaged plaster wall to dry completely.
Water-damaged gypsum can have serious health consequences and is often indicative of potential damage that could damage the overall integrity of the structure. Over time, water can be extracted from the masonry of the foundations (or inward from the outer stone walls) and damage any plaster applied to it. Paint and water are insoluble and an easy way to identify if the plaster is damaged by water is to look for paint in flakes or air bubbles that explode and flake off. As homeowners assess the damage caused by Hurricane Ida, it's important to remember that many historic materials, such as gypsum, are resistant to water damage and can (and should) be repaired.