http://en.wikipedia.org/wiki/Laminitis

Some good information there on the causes and hows and whys. Just if anyone is interested.

Kimmy is right it can be caused by uneven weight ditribution, among other things. When Barbaro got an infection in the right hind from the plates, he was in pain and put too much weight on the left hind, causing the mechanical failure and laminitis. The biggest threat now is for him to get laminitis in another hoof, that would probably be the end, so they are trying to get him pain free again and using the sling to keep him from leaning too heavily on any of the other feet. Thankfully he's doing better now.

http://www.naturalhorsetrim.com/Section_10.htm

The above link is more in depth and has pictures (NOT PRETTY) at the bottom.