Clancy's style really didn't change, just the volume of what was written.

There's a big difference between the spin-off books where he probably discussed the creation and backstory but otherwise just lent his name (e.g., Tom Clancy's XXXXXXXX) and the books he published under his own (and ONLY his own) name. I think the difference you saw in his later writing was that he was adding more and more detail and the editor was probably more reluctant to trim one of the best-selling authors in the business. Frankly, I thought "The Sum of All Fears" was too damned long by about 300-400 pages although I'm not sure what I would have cut -- probably a lot of the backstory behind the jihadists.

I would bet anything there was a ton of backstory material trimmed from "The Hunt for Red October" and "Red Storm Rising" -- Niven and Pournelle once estimated that only about 10% of what they wrote really made it to print -- whereas Clancy likely insisted on keeping a large portion of what he wrote for "The Sum of All Fears", "Without Remorse", "Debt of Honor", etc. because by that point he was in love with his characters and he knew his fans were too.