What's the best start to a season for a team that went on to get much worse and get relegated by the end of the season? In the EPL, Hull City is currently 15th (tied with 16th place Blackburn on points, games played, and goal differential and Hull City is ahead with 1 more goal scored if that is the next tiebreaker like I have sometimes seen) and 4 points above 18th (the top relegated team), but at https://www.bigsoccer.com/forum/showthread.php?t=965090, kevbrunton predicted they will get relegated and with 3 of their remaining 6 games against teams currently in the top five relegation is possible. Hull City is at Sunderland next and Sunderland will pass Hull City with a win. Hull City got off to a great start with 6 wins (including at 4th place Arsenal), 2 draws (including hosting 6th place Everton), and only 1 loss albeit a 0:5 home loss to 10th place Wigan Athletic. Hull City's tenth and eleventh games were acceptable losses, 0:3 hosting Chelsea and 4:3 at Manchester United. That started Hull City on a run of 1 win, 6 draws, and 11 losses in 18 games which is almost half the season. That ended with a March 4 win at Fulham, and after that Hull City has two losses and two draws. If Hull City barely survives it will be thanks to their great start and the opposite of Fulham barely surviving last season due to their great finish.
Millwall were top of Division One (now the Championship) at the end of January 1996 and were relegated that season.
Fiorentina in the 1992-93 Serie A season was as high as fifth or sixth until Luigi Radice got sacked nearly half-way into the season. They possessed players like Brian Laudrup, Stefan Effenburg and Gabriel Batistuta. Fiorentina did lose 7-3 to Milan at home but the loss that really did it was a 1-0 loss at home to Atalanta. Apparently Cecchi Gori didn't like Radice's zonal defence structure and he went for Aldo Agroppi. As a result, they sunk into the relegation zone, Luciano Chiarugi tried to save them but they were relegated on the last day despite beating Foggia 6-2.
This thread is about teams that started well in a season and then fall apart. As for rises from Serie B to Serie A, nothing beats Vicenza in 1977-78 when they got promoted and came second. Back on topic, Foggia started well in the 1994-95 season and were looking at UEFA Cup football. After the Christmas break, they struggled, got relegated and they haven't been back in the Serie A since.
That was due to a ruling, not based their performances on the field even though you are technically correct. Juve didn't go down due to numerous defeats so it's not like the other cases mentioned in this thread so far.
In France, in a recent past, Lille was 4th after 16 matchs in the 96-97 season and they finished 19th. I have to seek for cases before 1989.
In 93/94 Norwich City won 9 of their first 22 games and sat in 7th with 33 points in January. They only won one of their last 20 and finished 20th. Pretty similar to Hull.