In the workplace, it is often said that there are several warning signs before being fired. Is the same true in congress? I would say the main issue is declining percentages if one's percentages gradually start declining, then it means that the next wave against your party or even a neutral year, you're toast.
For a lot of the unseated incumbents this year, many of were only in their first or second term and had never won by an extraordinary margin so it wasn't much of a surprise. For the older members, a lot of them were out of nowhere and some of them weren't that unexpected:
Chet Edwards had only gotten above 60 percent of the vote three times in his career (1992, 1996 and 1998) and his luck was about to run out sooner of later
Kanjorski ran six points behind Obama in 2008 so it wasn't surprising to see him lose.
A few others were more unexpected
Boucher had never had a tough re-election race since the 80s and had run unopposed previously so there was no warning sign
Ike Skelton had always won above 65 percent and the only close race he had was in 82 when his district was merged with a republican congressman. His percentages hadn't been declining so it was probably a shock that he lost.
Boucher-his district was getting more Republican though. 2004 PVI: R+7 2008 PVI: R+11. I think he voted for Cap & Trade too and his district did not like that. A district like that just screams conservative in that its a Virginia district in the western part of the state by the Tennesee border I believe. So basically there was alot of factors at play when Boucher got defeated.
Skelton: I think his district was Republican to start with and he had a good opponent when he lost to Vicky Hartzler.
Well Boucher had fallen below 60 percent only four times in his career 1982 (when he unseated an incumbent), 1984 (51%), 1994 (58%) and 2004 (59%) so it wasn't like his % were declining
Skelton had only fallen below 60 percent twice in his career: 56% in 1976 and 54% in 1982. I'm not sure if he had ever had a strong opponent but I always thought that he was formidable.
A better example would be someone like Vance Hartke of Indiana:
He was first elected in 1958 in a good democrat year in an open seat with 56 percent. In 1964, as the Goldwater brand proved toxic, he won 54 percent and ran about 1-2 points behind Johnson. In 1970, even in a midterm of a republican presidency, he won by only a couple hundred votes and that was after it was discovered his opponent had lied about military service. By 1976 he was SOL.