htaccess randomly changes since 3.7 upgrade

I run about 20 wordpress sites for my clients and all of the site have crashed at random times in the few weeks since updating to 3.7

I have been able to fix most of the sites by restoring the htaccess file to the default WordPress code…

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

Some of the sites are still randomly rewriting the htaccess file. Usually adding random letters or cloning sections of the code. This immediately crashes the site.

One site is doing this almost daily.
http://tennysongroup.com.au/

I have been running WordPress sites for several years and have not seen this behavior before.

Is there any fix?