Hi Endeavour,
I don't know if snakes can deliberately/intentionally choose to inject venom.
However; I remember discussing this subject with a few friends a couple of years back and I remember someone saying that they read a paper on the ejection of venom by vipers and elapid snakes, (rattlesnakes, lance heads and cobras) where it stated the injection of venom has got a lot to do with the lower jaw gaining enough purchase on a victim to provide a pivot point for the muscles at the back of the upper jaw to be able to clamp down hard enough on the venom gland to force the venom out of the glad, through the fangs and into the victim.
So if this is correct as I assume it is, it would be a reasonable assumption that dry bites occur when the lower jaw of the snake has failed to gain a strong enough grip to cause the back muscles of the upper jaw to apply sufficient pressure on the gland and hence, force venom through the delivery system and into the victim/prey.
So in saying that it can be further assumed that dry bites may happen when a snake "snaps" in a defensive or instinctive action rather than delivering a bite where the intention is a deliberate act to inject venom and immobilize the prey or assumed threat.
George.