This can happen anywhere with any commandButton, not just PrimeFaces.
If you don't have a valid navigation destination specified,
JSF will select the default destination, which is to redisplay the current view.
Also, if you have one or more invalid input control values on the form being submitted, JSF will short-circuit the value-update and action processing phases of its lifecycle and redisplay the current view. Normally you'd have placed some sort of error message display tags on the view definition to display what the problem is.