The Official Story. As the seventh adventure in a path that ran through G1-3 (1978) and D1-3 (1978), most folks expected the finale to be by the same author — which is to say Gary Gygax. Instead, Dave Sutherland was brought in to finish up Gygax's epic. In a preface to "Queen of the Demonweb," Gygax explained why: He said that "The Temple of Elemental Evil" (then also unfinished) had kept him from finishing Q1 because they were too similar in nature. He also praised Sutherland's work to the outer planes, saying that he'd personally chosen to put the adventure into Sutherland's "capable hands" and that the result was "a superior design" and "a fitting climax" (to the adventure path).
The Real Story. It turns out that Gygax was putting on a good face, and that Q1 was in fact produced against his express wishes, apparently at the insistence of Gygax's partner, Brian Blume. It would be the first outward sign of a division in the company that would lead to its takeover by Brian and his brother Kevin in 1982, and Gygax's subsequent exile to California (where he'd get the Dungeons & Dragons cartoon on the air).
If you look closer at Q1, you can see the cracks - i.e., that it's not actually the sequel Gygax intended. A careful reading of the G- and D-series modules suggests that the villain of the piece was the Eilservs drow clan, who worshiped the Elder Elemental God. If anything, Lolth should have been an erstwhile ally for the player characters in a final adventure against the EEG, rather than a final foe.
Gygax said that the villain got changed after Sutherland discovered the "demonweb" pattern in a hand towel and talked Blume into making Lolth the Big Bad.