Boba Fett fell into the the Sarlacc Pit in Return of the Jedi due to the rather clumsy actions of a blind Han Solo and being on Jabba the Hutt's barge in the first place.
But why was Boba still in the company of Jabba the Hutt? He's a bounty hunter and one that has no compunctions with working for anyone, be they Hutt or Imperial.
Why did he stick around and wait for the rebels to come mount a daring rescue?