There is one other possibility. If the author had a backlist that was selling pretty well and changed publishers, the old publisher might tank on the last novel out of spite or to keep him from getting traction at the new house.
This brings up another interesting question: To what extent does the tone of your book cover reflect the tone of your book?
The cover of "Tricycle" is basically in the style of a "Cracked" magazine parody cover. But the story itself, as I recall, is without a molecule of humor. It's basically contemporary gothic horror-suspense in the vein of John Saul's many "punish the sinners" novels (anybody remember those?). The cover promises a much different story than what the contents deliver.
Right now, I'm reading a new mystery — "Deadlines," by Paul McHugh — with a cover that screams "contemporary issues non-fiction." It's a minor distraction, but a distraction nonetheless.
Of course, almost every authors' conclave is incomplete without an anecdote or two about how one publisher or another forced an awful cover down the author's throat because the marketing department thought it would grab the most co-op eyeballs. Almost every author I know say they get, at best, cursory, courtesy input on their covers. Does your mileage vary?
Here is a cover I designed for the novel I am trying to sell now. I have put together a few chapter of it for testing with my friends. I thought it would look better than handing them a stack of papers.
No. You're quite right, Jim. They do offer the courtesy of sharing the design with me before the book is released, but I have no input. A recent squabble over a French cover made me so angry that I won't post it on my web site. In my case, historical inaccuracy is frequently the problem, but there have been others.
I should add that, by and large, I don't think much of the average mystery cover design.