htaccess file keeps geting modified

my htaccess file randomly keeps getting modified and site becomes unavailable.

I have Sucuri turned on, and no user is logging in. It shows that the system is modifying it.
i have all plugins & wp up to date.

If i delete modified file, the site comes back up.

Below is bad one with extra code:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ – [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress
ST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ – [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress