Hi Crispy,
As you gain experience you will get to know what sets them off eg. Weather, end of flow, queenless, poor handling such as crushing bees, inspecting too often and so on. Also, as your hive gets stronger you have more foragers. Foragers tend to be more agressive than nurse bees.
All hives have grumpy days. Some have more grumpy days than others and those are best inspected on perfect days.
For me. If I cant walk past a hive without being harassed or stung then I requeen, or if every inspection is a nightmare and I dont enjoy handling them I requeen.
One lesson I learnt early....if your gloves have been stung wash them before next inspection as they become a target. If you get stung on your gloves during an inspection smoke them. I use nitrile gloves for this reason.
Give the girls a chance. Next inspection pick a perfect day and handle with care.