Red Hat Enterprise Linux Server release 6.8 - vsftpd dead but subsys locked

**Server
- Linux LGDFMODFSH1 2.6.32-642.el6.x86_64 #1 SMP Wed Apr 13 00:51:26 EDT 2016 x86_64 x86_64 x86_64 GNU/Linux
- Red Hat Enterprise Linux Server release 6.8 (Santiago)

======Symptom=======
- FTP services working stand alone
- memory status
total used free shared buffers cached
Mem: 15G 12G 3.2G 252K 1.4M 46M
-/+ buffers/cache: 12G 3.2G
Swap: 15G 1.8G 14G

• Issue event: vsftpd dead but subsys locked

• Action:
  Delete file /var/lock/subsys/vsftpd
  start service vsftpd

====================
**Question:

Have any body knows the reason why my service ftp get that issue ?