What had happened was that the first was unexpectedly successful and the cast was begging for sequels. Pre-release, Disney was unsure of its box office potential but once the numbers came it, the sequels were greenlighted (of course after Depp was last to sign on for them).
However, the problem was that the two screenwriters were off working on separate non-Disney projects. Once the sequels were ok-ed, the screenwriters were bum-rushed back together and to write a script. They wrote bits and pieces; tinkering out the plot and trying to smooth it out. What happened? In the middle of all this, Verbinski and co. were starting to do casting calls. Disney was also responsible for pushing the start date for filming as soon as possible (care to guess why? ) Verbinski was also forced to determine the budget for the film within a given deadline but it was difficult to do so without a completed script.
This all came back at the writers, who basically was getting the whip cracked at them repeatedly to finish the script. Even when the budget was decided, the casting calls, especially for Tia Dalma, was difficult because Verbinski and the casting crew was using work-in-progress parts of the script that the writers were in the middle of rewriting.
That said, I actually liked all three films. The last two needed to be one single plotline; either settling debt with Davy Jones and a father-son reunion or the advent of capitalism/imperialism/globalization with Cutler Beckett. It was far too dense. It's no wonder people got confused.