I wheelie all day every day. and the length of you're wheelie has nothing to do with it. It's how you land. More than likely, he's doing pop-a-wheelies. Rippin them up, and either droppin instantly or runnin to redline and slammin it back down. This is what causes the blown fork seal. That along with maybe a bad seal to begin with, excessive wear, dirt, and so on and so forth. But more than likely, it's rediculously hard landings. When doing wheelies, you always want to accellerate a bit as you land. This keeps you from landing hard and also helps with tank slappers.