Modell lost the team because (i) there were a ton of expenses he didn't anticipate - like buying out his minority partners, paying $23M towards the stadium and paying a $29M relocation fee - it is said that the move cost him $185M - and (ii) unlike very many owners at the time (and any current owner), the football team was his sole source of wealth.
The sale was technically mandated because his various loans were in technical default (did not meet operating revenue to debt service covenants) and while he got a $85M loan from the NFL to tide him over, it was only for a year and he had to find equity to meet the various loan covenants. However, it was clear that there really wasn't anyone willing to invest that kind of money for a perpetual minority interest.
Also, IIRC, he was trying to do some estate planning so that his family could keep the team.
http://www.baltimoresun.com/sports/ravens/bal-modell122099-story.html
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