Pretty simple. Phillip Fulmer was pushed out and the program tanked as they made bad hire after bad hire and never replaced what he was for the program.
Won the SEC East five more times after that 1998 National Championship—four double-digit wins in a good conference—but simply wasn't good enough to beat the likes of LSU or Auburn when getting to the big conference title game. Had some up years with the occasional down year, but fans pressured the AD to run him off (which works in SEC country with big boosters) and that was it.
Someone above mentioned if there's something for our fans to learn; absolutely.
Whether it's running a guy off because of inflated expectations, or chasing down the next flavor-of-the-month coach (in this case; Lane Kifffin)—the grass ain't always greener.
Not saying it was or wasn't time for Fulmer to go—but Kiffin, Derek Dooley and Butch Jones weren't the answer—while Jeremy Pruitt; another big name up and comer type, certainly has his work cut out for him.
If Tennessee was smart, they'd have hired Butch Davis after pushing Fulmer out—or in 2012 after they punted on Dooley. Davis may not be the best game day guy, but he recruited like a beast and could've built a solid SEC team as he was about 59 or 60 years old when Dooley got fired.
... and yes, Tennessee fans' egos are out of control—as is their sense of worth in the landscape of college football, which is something some Miami fans need a hard check on, as well.