It is an expression to indicate the advantage of acting when the adversary or opponent least expects it.
For me 'surprise factor' does not become a locution, and it is understood very clearly. I still explain, just in case I'm wrong: it's the factor, fact or part of an operation that some involved party doesn't expect, so it's surprising.
SURPRISE FACTOR In war : element that grants an advantage to the attacker, which is to surprise the enemy, attacking him so that he is unprepared.
It is said this way when you are going to use an unexpected method for the people you want to surprise.