I'm with Xalik on this one. You choose when to start a fight. Normally, if your opponent is surprised, which he/she/it should be if you are hidden, you get a partial action. For your partial action you decide to forgo your action and to delay.
Effectively, you discard your advantage of (possibly) attacking twice in a row to remain hidden.
At first I thought that "partial snipe" should be an option, but then I realised what sniping was: a slow, hidden, attack. You, slowly, peek behind a pillar, take aim, shoot, and go back to the shadow.
You cannot do that fast. And to ensure you cannot do it twice in a row, you don't get partial snipe.
The real question is, can you load a crossbow while hiding?