Probably cultural. Where I'm from (NZ) chairs sorta belong to the table; you wouldn't take a chair from a table or hang your jacket on it if there's someone else sitting there, at least not without asking first.

I had a similar experience in a food court in Bangkok. My wife and I bought our food and sat down at a table that had some bagged food on it (since there weren't any clean tables available). Assuming it was someone's leftovers that they neglected to trash, we just moved the food to the side since there wasn't a rubbish bin nearby.

A few minutes later someone came up and angrily told us that was his table and it was his food, and he demanded we find another table.

Really confused but not yet learning our lesson we moved to another table and moved some more food aside. Another man yelled at us, this time even angrier.

Seems it's pretty common for people to buy one thing, take it to the nearest empty table to 'reserve' it, and go elsewhere and get more food.

I guess when you're in someone else's country you just have to realise that you probably are the asshole and apologise, even though it doesn't make much sense :S

Maybe he was alone, he went to buy a drink and came back to find his seat taken? I wouldn't be too surprised if leaving your jacket to claim a seat is a common practice among older generations. If I imagine the setting to be 50's/60's, and watched it in a movie, it would seem you were the assholes. At least offer him a space in the table to join you. Alas, this is the modern times, so my critique does not apply, it's only a suggestion why the older man might be angry.