I've heard a conspiracy theory that the NBA is as fixed as the WWF.
By whom? Organized crime (bookies, vegas).
How? Bribe or threat. There's anecdotal evidence regarding Jordan along these lines: Jordan makes freethrows at the end of games with his eyes closed, yet misses two in a big game (big money wagered) so the Bulls don't cover the spread; Jordan owes gamblers money from golf debts; the death of Jordan's father, etc.
More "How"... A game can be thrown by a player or a referee or a coach. The conspiracy theory goes that it varies from game to game, they're all in on it, and when someone does try to "rat" on it, they're taken as lunatics or disgruntled...
Why? The theory goes like this. There's a heierarchy of bookies. People bet with the local bookies. If there's too much money on one side of the bet, the bookie is at huge risk, so he bets the other side with a bigger bookie, so he breaks even. This works its way back to Vegas, where the biggest bookies of all are (casinos). The casinos get a bird's eye view of all the wagering and they set the lines; often they'll set the lines to strongly encourage the bettors to bet on only one side. Fix the game so the other side wins, and Vegas cashes in. A few $million on every game. We're talking about winning bets, not games - if the Bulls, say, are favored by 15 and they win by 14, betting on the opponent wins.
The theory goes: NFL, NBA, MLB are all fixed.
Do I believe it? Not really, and you won't either.
However, it does make you wonder when you see a guy dribble the ball off his foot at half court, pass up an open shot, or miss critical free throws. In the NFL, it would be easy to cheat (throw a game) - the QB doesn't pass, gets sacked; receiver steps on the sideline with an open field in front of him; the refs either call things close or allow one team to hold at the line of scrimmage - think of how many replays you've seen of a receiver running around in the end zone waving "throw it!" and the QB somehow doesn't see it

In baseball it's the easy grounder that goes through the fielder's legs, bobbled balls, not hustling on the bases, etc.
I have a friend who lives in vegas and bets sports. He's used this theory to win a LOT of the time. One thing to consider is that to bet according to the theory, you have to deduce which side of the bet the money is on and bet the other side - this is rarely an easy thing to do. I'll also point out that the effect of using this strategy is really to be a contrary bettor - most people lose betting, so if you're contrary to "most people" you're going to win.
There's another angle to all of this. If the league is aware of any fixing, they can get involved in the fix, too. There's lots of TV revenue for each game beyond 4 that a 7 game series goes.
Probably just B.S., but fun to think about.