Re: where did it come from?
The timing is right for it to have requeened itself, even if you did not see a queen cell. Other option is that a strong swarm moved in, basically took over the hive. As far as pissed off, they might be on the hungry side and I doubt they much cared for the tractor. Most of the time I can mow alongside my hives without a suit, unless they are not being fed well during the summer.
Thankfully, the bees are smarter than I am. They are doing well, in spite of my efforts to help them.