The Sucuri plugin only opens the main access control file (the one in the document root) for reading, never for writing. The only files that it opens for writing are these [1][2][3] and only when you go to the plugin’ settings page, Hardening section, and click one of the buttons to block the direct access to the PHP files, it doesn’t modifies these files automatically, only per request when you click a button.
You said this is a recurrent problem, if I was you I would disable/delete the Sucuri plugin for 1-2 days and see if the problem keeps happening, if yes then you will be sure that the culprit is one of the other plugins, if the problem stops then I guess you could blame the Sucuri plugin, if this is the case, please contact me again so I can continue the investigation.
Marking as resolved for now, feel free to re-open if you need more information.
[1] /wp-content/.htaccess
[2] /wp-includes/.htaccess
[3] /wp-content/uploads/.htaccess
