Yeah you'd be surprised what carp will eat somedays..from crayfish to leeches to anything that strikes their fancy sometimes. Or it may have just been unlucky enough to swim by lol..real strong fish though, lotta fun.
Your not wrong, as far as manners and etiquette go. While no one owns a swim or spot, unless they do indeed own it heh, it has always been a bit of an unwritten rule that fishermen will give each other space to fish ad not infringe on each others spots, even if they are hot. We anglers have always had a certain respect for each other and while, competitive by nature, have striven to maintain boundaries. As in anything there will always be assholes if you will though, pardon my language but it is early and I've not had coffee yet
That said, manners in society ( and yes this is a generalization on my part based on my experiences over the last few years specifically) have been on the decline. I constantly have to get into altercations, minor ones anyway, with people who feel they can simply sit right next to me and proceed to cast over my lines. Or who think they can simply walk up to me and I owe it to them to teach them every bit of what I've learned...
Simply put, in my opinion a bit of manners go a long way. A simple
"Hi, how are you. Catching anything?" instead of just
"What you using? How you make that?"
In my opinion they should have slowly passed you, maybe ask you what you caught and how the days has been..then given you at least some breathing room if they were going to fish the same area..and by no means have cast anything into the swim you and your pop were fishing. To have stopped in front of you or even anything resembling close to you and start casting, without even asking if you minded, is indeed rude.