EWWW IO integrates with the WP_Image_Editor class in order to optimize any images that are generated via this class automatically. It is most often used for resizing images, but can also be used for image rotation, scaling, etc. Some plugins will even use the class to dynamically generate images that are the correct size for display on a page, if the current image is too large. This sounds great, but if the plugin does not properly check for a cached image, it will attempt to regenerate the image(s) on every page load. This, in turn, results in EWWW IO re-optimizing those images on every page load.
To help prevent this, EWWW IO will check the filename passed to the WP_Image_Editor save() function to see if the file already exists, and is the same size. If the file already exists with the same size, it will prevent the new image from being written to disk, and will use the existing image instead. This also prevents optimization from recurring.
If, however, this is causing problems on your site, you may disable it with the EWWWIO_EDITOR_OVERWRITE constant. You would add this line to your wp-config.php:
define( 'EWWWIO_EDITOR_OVERWRITE', true );