What did i do wrong to be yelled at?

I have been living in Germany for two months now (I came from the Middle East). I am studying for my master’s degree here, and I have a C2 level in German. I think I have a good understanding of the culture and have also made many great German friends at university.

One day, I was at the gym. In the locker room, there are many benches, usually shared by two people at the same time to place their bags or other belongings.

I saw a bench that was half-empty. There was a bag on one side, and a young man was changing clothes on the other half. I assumed the free side was available, so I sat down and started taking out my shoes. Suddenly, an aggressive man appeared and yelled at me in German: “You saw me sitting here, didn’t you?” (He used duzen instead of siezen, which felt very direct.)

I quickly realized that the bag belonged to him, not the young man. I apologized and packed my stuff as fast as I could. To be honest, I had seen him using that side of the bench earlier, but I thought he had left.

Now I’m wondering: Is it normal for someone to get so angry and shout at others over such a small issue?