That's bees for ya. Some days they're calm as ever, other days it's armageddon.
I like to do my inspections either in early morning when it's a little cooler, they seem less agressive. Or mid-day when most of the workers are out of the hive.
If I open a hive and they seem upset, then its time to just close it right back up and try again tomorrow. Just like people, they have their bad hair days and are grumpy. Let them sleep on it and things will be better the next.