Stress and frustration are almost always the source of seemingly inexplicable anger. I'll use myself as an example as I was guilty of it just last week:
We're massively short-staffed at work (maternity, illness, one bloke getting run over by a customer, the usual stuff) leading to too few people trying to do too much work. I'm moving house which is always stressful but additionally many of the things that would normally go smoothly have gone badly - tradesmen not turning up, furniture being damaged during transit, etc. And to top it all, my dad's been in intensive care all month.
Last Thursday I went down to Brighton to visit him. It's not exactly a short trip and since I don't drive, I had to use public transport - given it's about 5 hours of traveling and I didn't want to visit him until I had two clear days in a row so I could have a proper visit rather than trying to make the trip down, spend 10 minutes with him, and immediately leave - and given the above, getting two clear days wasn't easy.
The train down to St Pancras was fine, but on the tube between St Pancras and Victoria some sort of problem occurred, leaving us trapped in a tunnel for what seemed like an eternity but was probably a little under 10 minutes. Still, it's trapped for that time in a hot tube carriage packed with strangers at a time when I really want to be making progress on my journey. The tube eventually pulled into the next stop and the driver announced that there will be further delays so I think "sod that" and get off. It's a reasonably short walk to Victoria; just through Queen's Park, round Buckingham Palace, and down the road. Only 15-20 minutes of walking and infinitely preferable to spending an indeterminate length of time in said hot, packed tube.
But as I walked through Queen's Park, a nice young man stepped out and asked if I'd care to donate money to his boxing club. Now under normal circumstances I'd offer a polite "no thanks" with a smile or if he was persuasive enough I might actually hear him out. But instead everyone around me got to hear me snarl a loud and very aggressive "no". To all those dozens of people milling around the park in earshot, it must have seemed like an outburst of inexplicable anger much like the OP reports.
But it's not inexplicable really - it's the stress and frustration from the house moving, the build up of work, and being further delayed whilst trying to visit a very ill father, and then some random lad trying to hit me up for money. Perhaps that man on the escalator was having a bad day. Or a bad week. Or a bad month. Sometimes - despite our best wishes - we simply can't remain cool enough to tut and grumble.
but mostly I find it a good way to become less stressed!
