A paid member only thread at WebmasterWorld has outstanding discussion around the topic of competitors stealing your links.
You have sites that you work on, continuously over the past several years. You build high quality content, attract relevant and top quality links and then your competitors just take it all away from you.
The thread discusses methods used by competitors to easily take away those links through manual means. These are not hacking attempts but rather scare tactics. The thread discusses specific tactics that work at really high rates. I won't discuss those tactics here, since it is a private thread but I will discuss the advice given.
It is always a good idea to keep track of your current links and then be aware when a link you acquired over time is lost. If it is lost, you should be aware of why? Was the page removed? Did they drop all the links? Did the link swap out to a competitor? So keep tabs on your links and, in this case, be reactive to changes with your links.
Forum discussion at WebmasterWorld (paid subscription required).