Actually, there is no specific criteria. Whatever you are not expecting may be deemed as a surprise pick for you. For example, I think Khurram Manzoor was picked out of nowhere for T20 WC 2016.
I don't think Sharjeel, Kamran or Sarfaraz can be termed as a surprise pick.
Also if Pak in includes a third spinner which isnt very likely but if one of Usman Qadir, Zafar Gohar or Umer Khan is selected it can also be called a surprise pick.
Zeeshan Ashraf. Not likely, but that's exactly why it would be a surprise pick for me. Could fill the wicket keeper problem without having to rely on players who are near retirement, and instead groom someone for the next 3 T20 world cups.
Also if Pak in includes a third spinner which isnt very likely but if one of Usman Qadir, Zafar Gohar or Umer Khan is selected it can also be called a surprise pick.