If the orcs are behind a rock, then it would benefit them more to use surprise AFTER the party has walked by the rock, and get a charge from behind.
If they're in bushes, remember that charging is no longer stopped by difficult terrain, it just takes longer. Therefore, orcs can charge from the bushes out towards the party as part of the surprise round.
In any case, the orcs jumping out did use their surprise round to jump out, as per the rules. They would be smarter to hide longer so they can gain a charge or ranged attack, not just a move.
If the PC's are aware, then surprise rounds are not granted (I.e. if the orcs jump out, then roll initiative, they jumped as surprise)