Swelling occurs when the ambient moisture level in your home changes. This is often found during summer months in homes with no climate control (no air conditioning). The wood is constantly trying to stay balanced with the humidity level of the room. If the Amish furniture is not sprayed with a film finish (conversion varnish) the wood will take moisture until it balances with the humidity level of the room, causing swelling.
Consumer note: One place the wood swells is the solid wood leaves of dining tables. This is natural and will change through the seasons. This is not an Amish Craftsman defect, just nature taking it’s course.
This is a basic overview of the woods that Amish furniture craftsmen use. If you would like more info check out your local States DNR website for specific tree details.