Maybe it was just a combination of a one off day, the traffic and the heat - who knows.
Think you have got it right there. People who have been sitting in traffic, regardless of what vehicle they're using, are going to be agitated. I detest sitting in traffic which is the reason why I use a bike for work all year round. There is no excuse for being an arse by deliberately blocking someone elses progress, but it happens.
I see other motorbikes filtering and I just cringe, diving in and out of gaps, aggressively revving their throttle for motorists to move, you want to wind car drivers stuck in traffic up? That's the best way to do it. There's no need for it, you're on a bike, you're going to get past them, be it this set of lights or the next, just calm down.
My experience of filtering in Kent/Essex/London (most places I've ridden regularly) have been mostly positive, but it's this time of year that motorists do seem to be a bit more 'aloof' with their driving, wandering over white lines as you pass, weaving about looking down the queue to see what the hold up is etc.
If anything I've had mainly positive experiences from white van man and Audi/BMW man, filtered to front of queue or can't progress any further, winds window down "That a blahblahblah mate?" "Is it fast mate?" all the usual car drivers questions
Got stuck in a bit of heavy traffic in Essex last year and end up having a thumbs up frenzy after filtering!