Amin,
The process instance will auto-complete if there are no more active node instances currently executing. In this case, once it reaches your first end node, it will find no other active nodes and will complete the process instance. Note that the signal event node does not prevent this, as that node is not active at the time and we can't wait for it as it might never occur.
If you always want to wait for the signal, you should model this using a converging parallel gateway.
Kris