When the tabletop was cut it changed the weight disbursement and design of the table. You might look at the brackets or mounts used in holding your shelf. Also what did you use to make your shelf with? Sometimes using a lighter shelving product is not the way to go. Because your table can lose some balance and stability. It might be as simple as adding a little weight to the shelf (try duct taping a wrench to the bottom of the shelf and then sew something fast, see if you still have all the movement).