For trunk racks, you need to ensure the straps are tight even after you mount the bikes. This includes the top, side, and bottom straps. You can also check them periodically to see if they loosen up while you drive.
If any straps come loose, you need to re-tighten them and even look for solutions to ensure they don’t come loose again. Hitch racks are often more secure, and you can wriggle the set-up to check for movement. There might be some minor movement due to spaces in the set-up, but it shouldn’t be much.