Hot bees have a lower threshold for stinging, not a good-looking hive body.

If you are getting a new set of bees as a package or nuc, it's not a fully grown hive. That means there aren't as many bees which are capable of hive defense in the colony - and anyways, it's all hands on deck to build up the hive.

But in the late Summer, your hive is all grown up. And every surface is coated with live bees. And there is often a dearth in the flow of nectar. And that means there isn't as much honey to ripen, or wax to build, but there are more opportunities to accidentally crush a bee... see the picture I'm painting here? Bees are available to defend the hive. And they have a lot to defend.

In the late Summer or Fall, it is normal for a hive to display defensive behaviors during an inspection which involves removing frames.

Acceptable defensive behaviors during an inspection include:
• bees crowding where your hive tool is pushing on frames, or where a bee has been crushed;
• bees tracking your hand as you wave it above the hive;
• bees flying at your face (up to 10 or so), especially after popping boxes apart or pulling frames up;
• bees in orbit around your head (up to 10 or so) during the later parts of the inspection.

All normal and common and not reflecting anything on your hive or you as a beekeeper. It can be minimized by your inspection technique.
It shouldn't escalate quickly into multiple stings all of the sudden, or tens of bees on your veil. See below for unacceptable defensiveness descriptions.

Kevin Inglin has a podcast, the Beekeeper's Corner, and in episode 175 he talks about euthanizing a colony that was stinging his family members and neighbors, tens of feet from the hives, when no one was near the hives.
https://www.bkcorner.org/euthanizing-a-colony/

He requeened this hive with a purchased queen, he's not in Africanized territory... but sometimes bees are really that hot. It's not you, it's them. Sometimes.

Minimizing the chances you will meet defensive bees

Selective breeding is key to having gentle bees. If a hive is hot, its daughters will be as well, and it will release hot drones to make future nearby queens hot too.

Chickabuzz queens are sourced from bees in my apiary who have shown that they are gentle under normal inspection stresses.

But sometimes it is you. All bees will be defensive if we inspect in a rough manner, meaning we act like a bear!

Avoid rolling or crushing bees.

Avoid triggering robbing with open feeding, frames left out for more than the duration of an inspection, or opening the hive for more than 10 min during a dearth.

Avoid banging, scraping, and other bear-like noises.

Some inspection tips for good karma and calm bees:
• Use a frame grabber to pull out the first frame only, and then use the hive tool to push neighboring frames apart sideways rather than to lift frames up;
• Put frames that you've removed into a spare box, rather than just next to the hive;
• Move frames at bee walking speed;
• Smoke the top of the frame gently before prying it apart from its neighbor, and before pulling it up out of the box;
• Get a board the size of half the hive as a partial cover over half of the hive;
• After the hive is full sized, you need only remove frames 1-4 to answer all of your questions.

Do you have overly hot bees?

Your hive may be unacceptably defensive if:
• Often when you walk within 10 feet or so of the side or rear of the hive, or 20 feet from the front of the hive, bees fly at you with... intent. You can tell when they mean to drive you away.
• As soon as you walk within 10 feet of the hive, you get stung or chased.
• As soon as you pop the inner cover over the honey supers, several bees fly at your with... intent, and others stay in orbit until you leave the vicinity.
• When you start to inspect the brood nest, more bees are in orbit around your head, and you are getting facebumps on your veil.

Most backyard beekeepers don't have these experiences. Unless...
• You crush bees with nearly every frame manipulation, and don't use smoke when inspecting;
• You are inspecting, or visiting, when a storm is literally on the horizon, or it is nearing sundown;
• There are robber bees actively attacking the hive;
• A nocturnal 4 legged pest is scratching at the hive entrance regularly; and
• The hive is getting ready to swarm in a day or so, or has swarmed.
• Or it is really a very defensive hive!

All hives have the capacity to be defensive, but the threshold that "must not be crossed" varies from hive to hive. Once that line is crossed, some bees stay defensive for longer. But all bees will respond to us as a threat if provoked by us acting like a bear - intentionally or accidentally.

This is why you must always were a veil - so you can run if you need to.

For example, if you knock over a hive. Sounds impossible? You might get out there and not realize the ground has shifted, put weight just the wrong way and... you'd better be able to run.

Remember that bees go for the eyes and mouth, and do not risk it - wear the veil when you will be taking the lid off, no matter how quick the job is supposed to be.