They must have had a bad experience - unable to secure chairs at some point so they are totally paranoid/selfish because they are angry about it and are not going to let it happen again.
Sadly from a UK point of view.......when travelling in Europe....it also becomes a contagious condition.......ppl see others saving then decide to do it themselves.
Also.....some people will never be satisfied unless they have seats where THEY believe to be the best spots. Have seen it in Minorca where a family of 7.....saved seven loungers in the sun......next to pool.......and a table with 4 chairs, parasol, and 4 loungers in the shade for when it got hot....GRRRRRRrrrrrrrr