More bus etiquette, I'm afraid.
When you get on the bus and the seats at the front are taken, move to the back! It is truly not helpful for you to take up a position next to the wheel wells — the narrowest part of the interior of the bus — blocking the progress of everyone else toward the empty part of the bus in the back.
Extra props to those who do this with their shopping carts or baby carriages. I can't imagine how anyone thinks this is a good idea. Now, I will recognize that people are just not very good about getting up and offering the baby carriage area to the person who actually has one of those. Half the time that that area is available, the carriage pusher opts to sit somewhere else and make a big show of pulling the carriage as close to the seat as possible, still effectively blocking the passage.
I have to say that when I get on a bus and find myself facing a bottleneck because two people, often with backpacks, have installed themselves on either side of the wheel well area, I am starting to push through. I know that I won't fit through the ten-centimetre space that is left, so I just push, disgusting them with contact with my oversized stomach.
If you don't like it, stop blocking the front of the bus!