I have less experience than you. Please do not take me seriously.
My small experience is starting to tell me that sometimes there is a concrete reason for mean bees. My most recent experience was due to robbing starting in my hives, but did not show itself for at least two days. Earlier this year when my bees got mean, I watched and found herons manning the hives during the day and eating them. I hung aluminum pie plates to scare them away. Even earlier, my bees had another mean streak and I found 'coons raiding bees at night. I set rat traps to run them off.
My experiences will be nothing like yours. Seek your own path.