Droppers and dispensers behave in the same fashion as pistons with block updates, such that when a block update occurs next to a powered dropper it will fire an item.

What I expected to happen was...:
Droppers (and dispensers) to behave like redstone lamps or other redstone devices

What actually happened was...:
Droppers and dispensers firing based on block updates causing them to dispense items at odd times.

Steps to Reproduce:
1. Place a trail of redstone
2. Place a dropper perpendicular to the redstone (not powered by it)
3. Place a block above the trail of redstone where it passed the dropper
4. Place a lever or button on that block and trigger it