Its not a disturbing trend. Its always been that way. You are given the option to support v rivals so its up to you to choose.
If it bothers you and you've not experienced it yet then wait until your rival is part of a T11 group and the massed ranks of supporters descend upon you. >.<
Like this lot, you mean?
PC scored in 1st minute, and were running rampant until the 10th minute, when an ! appeared next to spectators - all this lot had arrived and apart from an 71st minute goal which was a direct free-kick, PC didn't get a look in...