They rounded up about 2000 bikes left behind this year. (yikes!) There are 50,000 people out there, so there are many bikes. Some people leave their bikes behind on purpose, but many of them were 'borrowed' by jerks and then dumped somewhere else. Plus the harsh climate tears up bikes, so if someone's wheel gets taco'd, or sand grimes up the crank, they often times give up on the bike.
There is a community bike program out there (see all the green bikes in the herd?), and they get first dibs on bikes to convert over to the program (single speed cruiser bikes). Then the rest are hauled back to Reno, and usable bikes are fixed up an donated to surrounding communities. Bikes that have no ridable value are stripped of useful parts and then scrapped.
There are all sorts of old and oddball bikes out there, and on more than one occasion I've wanted to find the owner and see if they wanted to trade it. There are thousands of (previously) new department store cruiser bikes that people purchased at the nearest walmart/target/ToysRus before driving out to the desert.
I volunteered at one of the bike repair camps out there, and assisted in fixing 40 flats, 3 bent wheels, 5 loose/broken seats, 12 derailed chains, 7 unaligned brake calipers, and other assorted issues, just in the span of 4 hours. Many people complained about the quality of walmart bikes, but I kept reminding them that walmart bikes aren't meant to be ridden on rough desert in 100 degree weather for a week.