A small part of the problem is a design flaw – the stock earbuds that come with any player are going to be shitty. They never fit anyone’s ears properly, so there’s always going to be leakage.
The larger issue is that people desire a cocoon. It’s a bit like the bass car issue, but in the opposite direction. The bass car is meant to attract attention, to say “I live with my mom and am probably a rapist.” A poem written in shitty music by shitty people.
The loud earbud issue is about isolation. If I cannot hear the world, it ceases to exist, at least until it’s time to get off the train. Whether this is a desire for a contemplative space in a hurried, secular world or simply wanting to drown out the hundreds of people who are also pretending that no one else is on the train…well, that is for each seeker to decide on their own. What it does mean is that you end up hearing a lot of cymbals. It’s also a bit of a blow against ethnic stereotyping, as (what sounds to be) cock rock is beloved by people of all races. When did that happen? Why did that happen?
While it is easy to give into bitterness, I prefer to play along, no matter how little it is appreciated. As with the Polish gentleman on a 6 train last year whose Eurotastic high-NRG grooves had him stomping his feet on the floor, I thought I’d play along and tried to lend my skill with a bit of (rather tight) counter-rhythm. He did not appreciate my contributions, but perhaps that’s merely a habit one picks up from being the leader of a one man band. An alternate explanation is that I’d messed with his cocoon, his little bubble of stupid, his obnoxious lack of care for the world around him. I pissed on his dreams of stardom, his masturbatory flight from his shitty life, with its regular wear and pressing concerns. If I’d been smaller he’d probably have tried to fight me.
And in turn, the rest of the world is merely messing with me and my self-absorbed bubble.
Sure, I only listen to music at 25% volume, and I wear in-ear canal phones that no one can hear, and I don’t stomp my feet or sing along. Not that I mind the singing along bit, as it is incredibly entertaining. What I do mind is how angry these songsters get when you laugh at their public pathos. It’s genuinely funny to see some guy wearing an oversized Fat Joe shirt singing some R&B song with the strangled, overwrought phrasing that Whitney Houston and American Idol have made popular. It’s funny because very few people can sing like that – an obvious fact to people with ears – but the songsters don’t seem to feel that way. Suffering from inflated self-esteem can be hazardous in the best of times; wanting to fight strangers for laughing at clownish public antics sits at the crossroads of comedy and suicide-by-proxy.
And so, smushed between two people whose earbuds were set to stun, this morning’s edition of “dueling hats” drowned out Asva‘s tones n’ drones. I couldn’t figure out a way to tap my toe to that even if I wanted to.
A long while back I’d thought it would be cute to print up business cards that read “Your taste in music is awesome! Thanks for sharing it with us!” A futile gesture raging from the inside of a ruined cocoon.
here there be rodents 
A girl at my gym the other night was listening to her headphones on full blast and would randomly burst into what sounded like bits of musical theatre intermittently. I looked over at her and she gave me a really angry look for messing with her showtunes cocoon.
another possible answer is that people forget they can’t hear themselves or what’s going on around them, and so react to any intrusion like someone’s sneaking up on them.
See, I kinda wanted to upload a song online with some segments filtered out above 11kHz, and pass out a business card with the address that says “Here’s what you’ll be hearing when you’re 35! Just like grandpa! Where’s my soup?!”
Having just started at one of NY’s prestigious audio production programs, I got to revel in the collective “Oh shit” that went up amongst the rap kids when we covered this very subject.
My personal favorites are not the earbud people though, but the white hipster douchebags with open-backed Grados around their necks. Note that you never see them trying to listen to these, cause in public that’d be impossible.