Deer can definitely jump the string. When they do, they are not ducking so much as coiling to leap away. It is a terrible (but amazing) sight to behold.

Nervous deer are string jumpers. Deer that are looking at you when you shoot can jump the string. I'm sure that loud bows amplify the problem, but even quiet bows can cause jumpers if a nervous deer is looking at you when you shoot. Deer are amazing athletes.

All is not lost, however. As others have stated, aiming a little low is a huge help. Top of the heart is a great hold, since a jumper drops into a lung shot and no drop is still a great shot. I've found that deer that are walking don't jump the string. Super close shots (less than 15 yards) can't jump the string. I've even heard (but wouldn't test or condone) that very long shots don't cause deer to jump.

I've found that a calm deer walking slowly at 7 yards and looking away won't jump the string. Good luck with that!