When accosted by a predator, rattlesnakes may stand their ground, rattle their tails and bite anything that gets too close. It's not a stretch to misinterpret these defensive behaviors as aggression. To an extent, the distinction is academic. Whether aggressive or defensive, rattlesnakes are capable of inflicting deadly venomous bites when they feel vulnerable. Rattlesnakes demand respect wherever they're encountered. At any rate, it's helpful to be aware of some of the circumstances that can make them defensive, or aggressive as it may seem.
Vulnerable and Venomous
No rattlesnake awakes in the morning and sets out to pick a fight. To the contrary, rattlesnakes are always wary of predators and seek to avoid encounters whenever possible. Two of their primary adaptations serve this purpose: Their cryptic patterns help them to avoid detection altogether; if this fails, their loud rattles help to keep predators at bay before the encounter becomes physical. Though they can deliver strongly dissuading bites; their venom is temporarily finite and takes some time to replenish once exhausted. Rattlesnakes need their venom for food acquisition; they do not use it defensively unless their lives are in danger. Accordingly, it is when rattlesnakes feel vulnerable that they are most likely to react defensively.
Blind and Blue
Like all snakes, rattlesnakes shed their skin periodically. While preparing to shed, they develop a layer of fluid underneath their outermost layer of skin. This fluid serves as a lubricant, helping the old skin to peel off. However, as snakes have clear scales over their eyes, the fluid covers their eyes for a few days. Herpetologists refer to snakes as "being blue" during this stage, as the fluid under the eye scale makes the eye take on a blue tint. This limits their vision significantly for a few days, usually causing snakes to spend this time hiding in a safe place. However, if they encounter a perceived threat during this time, they are more likely to react defensively.
Stuffed and Sluggish
Like many other snakes, rattlesnakes usually eat large, relatively infrequent meals. While rattlesnakes are well-adapted for this lifestyle, they have reduced mobility when they're carrying heavy meals in their stomachs. This reduced mobility makes many snakes feel vulnerable -- and more likely to stand their ground and fight than to try to escape.
Postpartum and Potent
Rattlesnakes give live birth to their young. Most stay with their young for only a brief time, they are quick to react defensively if threatened. Most wild rattlesnake litters are born in secluded places; it is rare for people to stumble onto a rattlesnake giving birth.