Otoharo!
Francisco Coll, in Inner Peace Movement explained this phenominon as this. Some people dump on others. The way thAey do this is as you describe telling another culprit, so to speak, all their difficulties simultaeous dumping their anxiety on them. We were taught, to stop them when they begin and tell them you do not want to hear it. Walk away if you have to, get up and leave if you have to. We spent one retreat event practicing this. When someone started, we simply said, excuse me, and walked away. Before I had said a word one time, I walked over near him, and he stopped a conversation with someone else and pointedly walk away from me. I got the point.
Another time, we found ourselves involved in a game when we entered the grounds for a retreat. Whatever we said to the first person we met (staff were doing the greeting of people) we were told whether we went to heaven or not. At the close of the event, we had learned about people's dumping their anxiety on others. Luckily, I went to heaven.
Is your question really, why do you take it on? Perhaps knowing in advance that this person dumps, Avoid them.
azalee