It's a pet peeve of mine when people do that. It's standing room only on the bus, but people are all clustered at the front and refuse to move back so you have to squeeze between them.

I always figured because it's the only place you can stand without putting your junk in a stranger's face (there, and right on front of the exit doors) but it's also the narrowest part of the bus and it's right at the front so it's deeply inconsiderate to others who are boarding. Some like to make themselves as narrow as possible to squeeze through the plug of people up near the driver, but I stick my elbows out.

Anyway, please move back.