.htaccess file hacked?

This strange thing keeps happening to my WordPress website.
Every one week my HTACCESS file gets modified by itself.
And the website keeps going down because of this.
The original HTACCESS keeps changing to this:

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

# END WordPress

from my original HTACCESS file:

# 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

after I upload the original HTACCESS file to my WP root folder, the website is working again.

For some reason this RRewriteCond %{REQUEST_FILENAME} !-f comes to my HTACCESS from somewhere, and that seems to be the problem.

Any ideas, how can I prevent the HTACCESS file to get hacked??
Thx in advance.