Tell me about it. I see dozens of them on my school's campus everyday. It boils down to three things:
1. Expensive failure. Most commonly seen in BSOs that litter the landscape. I've seen bikes that seemingly bent a wheel in strong winds. Of course, being BSOs, it's usually cheaper to just abandon them in perfectly good parking spaces.
Extremely relative. Some will dump $$$ into fixing BSOs while others will abandon them.
2. Stolen components. Common, but less so than #1. I've even been a victim of this, but I came back for the bike with a new wheel. Word has it somebody actually abandoned a Cannondale R400 (sold at the school's auction) because the rear wheel was stolen. Second-hand gossip, but I had the good fortune of buying a Marin Larkspur frame that was stripped down to the cranks in my size for $5 (legally, of course). So it happens.
3. The bike is unnecessary or they rider has bought a new one. I have friends who basically leave their bikes on campus because they don't like to ride home and back. So these bikes may look abandoned, but may not necessarily be. They are usually in poor maintenance, though.
Of course, there are the WTF reasons. One of my friends, for instance, abandoned a brand-new cruiser bike because he had nowhere to put it during the summer. It was an Electra bike, so a step up from the gas-pipes.
Then there was the time I saw perfectly good Cannondale V500 abandoned for no good reason as it was parked outside the same building in the same space day in day out. Of course, as I didn't see any notices, I assume it was either taken home or sold to a good home.
The transportation department does a bi-annual auction where they make some serious cash off this crap. You routinely see Huffy and Magnas go for $100. There are the odd Treks and Specialized bikes, but most people are smart enough to take them home. 'course, being the ...questionable sort they are, sometimes you wonder if these bikes really were abandoned to begin with. I quite clearly remember seeing a "WARNING! THIS BIKE MAY BE ABANDONED, IF SO, WE WILL CUT THE LOCKS ETC ETC" notice attached to some clearly not-abandoned bikes including a pair of brand-new Bianchi fixed-gears.
